R50 V2 - The standard V2 typically has a clutch bell with 2 grooves cut in the side though I understand the last run of V2 kits has the Titan clutch bell. Also has a standard length black tail tube, a single connector for the elevator and collective servo and if you have the original stickers, they are purple-ish colored.
R50 SE - The SE is the same as the V2 but has metal head parts, CF style windshield, CF tail fins
R50 Titan - Titan has a longer "Titanium" colored tail boom, boom supports and skids. It comes with a Push/pull elevator and (optional to install) P/P collective mechanism. The clutch bell has 3 grooves cut into it's side.
But really any of the three share 95+% of the parts. They are all interchangeable with the exception of the boom/belt. You need to make sure the boom and belt match in length - i.e. get a Titan belt if you get a Titan boom.
Also, be sure and pick up a PV0499 if you haven't already done so.