One thing to consider is which Mackie's are still in production. I believe that of the three versions, there may only be one still in production. You may also want to be aware that projects are not completely sharable between the SDR and HDR versions, so mixing them may turn out to not be so useful. The SDR also does not allow the integration of a mouse, keyboard and monitor, so you will not be able to edit in that same fashion. One nice thing about the SDR, and the Alesis HD24 is that they both come stock with the full compliment of I/O. They each have 24 analog inputs and outputs, and 24 channels of ADAT I/O whereas the HDR I believe does not come fully loaded, so add on cards must be purchased. The mackies also use DB25 plugs for the analog I/O. Personally, I like this, but if you already have all the cabling you need in the form of TRS, that could prove to be a deal breaker.
The Tascam machines are really nice as well, but if I remember right, are considerably more expensive than the Alesis and the SDR. The alesis also offers two removable bays. In many ways I really do like the Alesis better, but the Mackies are just as capable as far as sound quality goes, and offers a slightly better routing system. It seems to me that if you can live without the standalone editing functions, the Alesis is probably the better way to go (or the SDR) as opposed to the HDR. However, if you like having the ability to edit directly from the machine then the HDR is probably the best bet. I would check with Mackie though to see what features you may lose if you pair an HDR with an SDR. You may be able to have all of the features with that pairing if you treat the HDR as the master, but I personally am not sure about that.