With a quad core ARMv7 processor running at 900mHZ, the freshest Pi in the shop — the Raspberry Pi 2 — is considerably faster than its predecessors. In fact, it boasts speeds of up to six times faster! It also offers twice as much memory, with 1GB LPDDR2 SDRAM.
The Pi 2 has the same form factor as the B+, too. So your B+ cases should still work. The board is also compatible with all of the ARM GNU/Linux distributions, as well as MS Windows 10.
But, just a few days after it was released, eager owners are reporting a problem. Taking a picture of the Pi 2 with a Xenon flash causes it to power down immediately. Covering the U16 SMPS with a bit of blu-tack seems to resolve the problem, however.