21851 is newer than 23017, but comes from different branch. 21851 is few days old MW 6.5 release (btw. there's even newer 21854, I will try update that soon), stable and simple release. No big menus, .NET is version 2, no start menu on bottom, no moving icons, no taskbar zoomer etc. Such WM 6.5 MS will officially release at 6.10.
23xxx builds are WM 6.5.1, with a lot of UI changes, but you'll face a lot of problems with older applications on them. 23045 is latest and the best of them, I'm running it almost 24 hours now, and it is working (and looks) great. I planed to post it today if I found time.
The decision is on you. Have in mind that WM 6.5.1 is in beta phase, so if you need all-working and stable OS use WM 6.5 (218xx) releases. But you'll miss the fun...
One more thing: I build my roms to be CLEAN roms, not LITE roms. Everything MS include as part of the OS is in them with no (or minimum) changes. There's no 3rd party applications except Samsung's necesary files to run the phone. You'll have to install all the applications you need.