Personally, I would go with the 604's on Toms. Currently in stock between my studio and my live rigs I have 604's, PG56's, beta56's, beta98's, 421's, Karma K-Micro's, vintage EV 408's, and have gotten rid of my D1's, D2's and D3's. Of all of those, the K-Micros are certainly the cheapest, and the 421's the most expensive.
The K-Micro's may be a good place to start, but I would only recommend that if you are not afraid of processing ( some fairly hefty EQ'ing and probably gating) come mixdown. The K-Micro's were surprisingly good sounding to me but certainly not for just any old person to throw out there and get a good sound. They can sound huge with the right post processing, but can also be nearly unusable depending on a combination of the drummers setup, playing style and dynamics.
The vintage 408's are my primary studio tom mics. They sound thick, but EQ nicely on the top end. They also have a great design for getting good and flexible tom placement with a low profile so they fit nicely on almost any kit setup. The combination of the clip style and yolkable mic capsule offers lots of good positioning options with low effort and standard stand or clips. Mostly though, they just sound good.
The 421's are a nice option and are definately usable for other sources as well. However, they cost a lot more, even used. In the end however, on most toms I actually prefer the 604's, 408's, beta91's and Audix D3 or D4's. The 421's also use a clip system that is pretty much the worst I have ever dealt with. I rarely feel comfortable putting one out unless I have gaff taped the clip to the mic. Even brand new ones seem to easily release form their clips. It is very common to see 421's that have been put back in their mic cases with the clips taped to the mic and the tape may stay that way indefinately. Also, the 421 is housed in a body that is long and bulky and can be a real pain to get placed properly on a drum without interfering with cymbal placement. Especially if you want a steeper angle on it, or the drummer has already canted their toms alot. At the very least, if you plan to use them on toms, get some mic cables with 90 degree female XLR ends otherwise your cable may extend the length of the already long bodied 421 by a good 3 to 6 inches.
The 604's seem to me to be a good balance in a tom mic. Fairly small, the clips are fairly versatile once you learn some tricks as to how they best clamp on to a tom, and experiment with turning them around on their clips. I like the plastic they use to build the clips because it seems to do a bit better with isolating the mic from the shell then some other brands, does not break, and can be used on most rim mounts, although some of the rounded ones or very thin ones can make it hard to clamp. In my opinion, if you can not easily get a good sound out of the 604's then the mic is not at fault but the problem lies somewhere else. I have no proof of this, but have been told by some well respected and trusted older engineers that the md504 that is the predecessor to the current 604 sounds almost exactly like a 421.
Shure tom mics.... The PG series is a decent low cost alternative.
The PG56's actually sounded a bit better than I thought they would. On toms they do a pretty decent job, but I still prefer 604's for not much more in cost. The PG52 however sounded pretty horrid in my opinion on kick drum. It was to the point where I would almost prefer a sm57 or 421 (which for kick I do not care for) on kick drum. The Pg52 worked pretty well for me on floor toms though (14" to 18"). The beta 56's sound a little better to me on toms than the PG's, but quite possibly not enough to warrant any real price difference since a little bit of post processing (whcih seems to be necessary with either mic anyhow) greatly reduces the differences. One thing I dislike about all of the 56's (PG or beta) is that they are very heavy for booming out on mic stands, and akwardly sized and designed for using on drum clamps. Also, the spring pivot assembly for the mic capsule wears out easily and quickly which makes stable placement hard to acheive. This is true of the Beta 52 as well. The shure beta98's are great little mics, but more expensive then the others (except the sennheiser 421's). The beta 98's are very easy to place, sound full and clear, and are pretty versatile as well. They will not sustain a tom hit as well as the other mics listed though (neither will the Karma K-Micro's which I forgot to mention). I have had more than one beta98 have its capsule ejected due to a tom hit. The capsule is black and about the size of two watch batteries so it can be very hard to find in a dark venue on a dark stage. Also, the cables wear out easily at the connection between the supplied mic cable and the preamp body. Once this begins to wear out it may not take much movement to create a quick short which results in a loud "bang" sound in the PA. It is nice that the cables are long enough that on stage I rarely need an extra XLR cable to reach the drum sub snake. What I have done to protect my beta 98's is allow the cable to stand up from the preamp body about 3 inches and then let it fold down to the preamp body and gaff taped it to the preamp body creating a strain relief. This way if someone trips on the cable or pulls on it their will be no pressure applied to where the supplied cable connects to the preamp body and then a greatly reduced risk of the loud bang sound in the PA due to phantom power shorting. The older sm98's also sound great but used even thinner little cables.
The Audix mics are ones that I am hit and miss with. personally, I do not care for the D1 or D2, but the D3's work pretty well for me on rack toms, and D4's on floor toms. I do like the D6 on kick, but not the D4 (which Audix intended to be a kick mic at first until they released the D6). I like the size and layout of the D series mics. The are easy to replace and pretty darned durable like the 604's. On the used market, the D series may be a great way to go
Hopefully some of this was useful. Of course this is all my opinions and their are bound to be plenty of people who may disagree, however I have used all of these mics in the studio (except the K micros which I keep forgetting to pull from one of my live packs). I have also used all of the mics on live shows. Altogether I have probably logged at least 200 hours of mxing time with each mic listed and easily 5000 or more mixing hours with some of the other mics on this list so I feel like I probably have a pretty good wide base of things to base these opinions on. Good luck and have fun