Let's remember that the Peavey xxx is NOT here yet.... won't have that until about May or so. Thus, I'm stuck with my crappy Marshall solid state and forced to turn it up almost over half just to breach the volume of the drums.
Also, you said that the amp sound will outweigh the ambient sound from the sides. That said, would it be safe to assume that i STILL could easily get feedback problems due to the fact that the ambient noise is present?
Sorry for all the headache guys, but a $100 to me is a lot of money, and I'd sure hate to blow it on something only to dive headfirst into feedback hell.....
Well, keep in mind that ambient noise isn't really the source of feedback - feedback is caused by a "loop" being created where noise being picked up from the microphone is being broadcast through your PA, and in turn the sound coming through the PA is being picked up by your microphone, and in turn broadcast
back through the PA, where it is in turn
again picked up by your microphone, repeat ad nauseum.
Obviously ambient noise is part of the problem, but another part of the problem is the mic's position with regards to the PA speakers. If the speakers are projecting away from the mic and the mic is facing away from the speakers (as is common in a sound reinforcement situation where you have the mains in front of the band facing out, and the mic facing towards the back of the stage, into the grille of the (forward or side) facing amp. So, unless you've got your mains facing directly towards your amp mic, which is positioned in such a way that it's going to pick up a not-negligible chunk of sound coming out of your mains, I think the risk of a guitar mic feeding back (especially given the seriously high output of a 100-watt tube head) is pretty minimal. My experience has been if you've got to worry about anything, it's going to be the vocal mics and the monitors.
As far as condensors in a live situation, I want to say either Paul Gilbert or Bruce Bouillet had some sort of condensor on one of their amps when I caught them at the last G3 concert, but that's about the only time I can ever recall seeing a condensor on an amp in a live context. Just about every other guitarist I've ever seen live has had some sort of dynamic on his cab.
The SM57 is probably the safest bet out there - once you learn how to dial in an amp correctly to be "listened" to through a microphone, and once you learn how to position said microphone, well... It's tradition more than anything else, I guess, but the 57 has been used on SO many guitar tracks over the years that it's very much become the "sound" of rock guitar. It's not the fullest frequency mic out there by any means, but the frequencies it does pick up accentuate what your ear wants to hear from an electric guitar.
Almost as importantly, not only is it a mic that does sound great on a guitar cab, they're notoriously bulletproof. If you've only got the cash to spend on one mic, you need to buy something that won't go on the fritz if iit gets dropped/has beer spilled on it/has a guitar fall off a stand and hit it/spends twenty minutes talking with the old geezer at the bar talking about how his LP has so much sustain that you can pluck a note go out to lunch, and come back and it's still ringing/etc. In short, you can do a lot of bad things to it and you probably won't reallly hurt the thing. A condensor would most likely be a bit more delicate.
As far as SM57 alternatives, the e609 is popular and has been on my "to-buy" list for ages, ever since I read an interview with Petrucci swearing by it and a 57 in conjunction on a guitar amp being the best guitar tone he's ever captured (hey, we're all guilty of a little hero worship now and then.

). Likewise, I've also got an Audix i5 that I go back and forth on - the SM57 is a little more mid-heavy, while the i5 is slightly more scooped and a little more open in the highs. It's subtle, but nice - if you play a lot of metal/heavier stuff, the i5 might be a good match for you, and will cost you about the same, maybe $10 more, than a SM57.
You use your BlueTube as a live pre? I've got one too, but don't gig with it. Granted that's probably because I'm between bands, but it wouldn't have occured to me to take out anyway, lol. That saiid, a buddy of mine uses his as a pre he runs through before going into
a Rocktron Chameleon, and swears by the combination, so maybe a little thinking outside the box here is in order...