The 2800 series will have far more functionality built-in, especially in the IPIPGW series. The 2600XM series (as well as 2691) has much of it, but I doubt those platforms will get the latest-greatest advancements. If you're wanting to run data + voice (and eventually video), the 2800 series will trump.
This configuration will permit you to run a full T1 of voice and UP TO a full T1 of data (possibly more, but shouldn't be less than ~1.2mbit/s even with all 24 channels in use) simultaneously. To do this on two remote sites, you will need 2 2T modules and a DSP module (PVDM2-48) (or just a single 2T module without DSPs if you're going VoIP out to your PBX/etc) on your main-site router. Your remote sites will need 2T cards + a smaller DSP module (PVDM2-32). This will give you a significant (and flexible based on usage) amount of data + a full PRI of voice out each site, trunking both PRIs to the main-site and either routing them out via VoIP (less demanding and less expensive on the router) or breaking them back out to PRI on the main-site. You can also run CAS signalling if you want all 24 channels (instead of 23, depending on your telephony interfacing). You could encrypt both sites as well, but if they're PtP, you shouldn't have to worry. Crypto will drastically increase your bandwidth overhead as well.
Good luck, PM me if you need some advanced voice+data PtP configuration assistance -- no charge whatsoever for relatively small setups like yours.
