Not an issue for a hobbyist, but the XBee and many other products from vendors interoperate via the IEEE 802.15.4 protocol standard. So you can mix and match, now and later. That IEEE standard also exists for use in the 902-928MHz and 868MHz bands (EU). Another XBee consideration is that many remote data access and control applications can be done with the standard XBee applications firmware atop 802.15.4. No added micro needed in these cases. There are surface mount versions of the XBee (smaller), but may cost more.
The RFM12B has a nice Arduino libraray but yes, its low cost and thus spartan. But not bare naked like the $4 on/off keying modules.
I live in a dense urban area and the RFM12Bs work just fine - my own protocols.