First of all, good luck finding any new receiver that is 5.1 only. Onkyo does have a new model that is 5.1 only, but in general 5.1 only models are few and far between.
- With a 7.1 model you at least have the option of using 7.1 speakers.
- If you get one that has Zone2 capabilities, you can have 5.1 in zone 1 and stereo in the other zone without requiring an additional amp.
If you know for sure that you never ever will want 7.1 speakers or a second zone of audio, then a 5.1 receiver will be sufficient - IF you can find one.