What must be taken into consideration? Budget!!
Also, you will want to consider whether or not you will be doing anything fancy that will require processing power. I have used the 3600 series with ~20 Mbps HSSI connectivity. They seem to do fine but we have found the processors to be somewhat slow (the processor actually changes depending on how many slots you have - I think it ranges from 40 MHz to 80 MHz between the 3620 and the 3660).
If your budget allows, there are certainly more powerful platorms but I'm not sure from what you have said that it would be necessary.
