[Touch Pro2, Tilt 2 Windows Mobile ROM Development] venomshot629:
Let me add a bit more to go in depth on the differences in the versions, if anyone cares to read :)

There are two (2) different "puzzle pieces" needed to complete ROMs:

The first piece of the puzzle needs to be from Microsoft. Microsoft has been updating its underlying system, which is called Windows Mobile 6.5. There are two different versions of this that they are working on:

The version with the start menu at the top left corner, saying "Start", is version 6.5. 6.5 is the (now) older style of Windows Mobile, which is not as *visually* nice, but is very stable. You will can see the differences when you look through the menus when changing settings and adjusting time zone, etc.. The colors do not have gradients, and are solid. This version is better navigated using the stylus as well when changing settings, because checkboxes are smaller, and not all menus and options are very finger-friendly. Also, if you use the default Windows homescreen in this version (instead of using the HTC homescreen with the tabs along the bottom) you will see that all homescreen are very small.

The newest, other version is 6.5.3 (or 6.5.x), where the start menu is in the bottom left, which is just a button of a Windows flag in a circle (like you would see in Vista or Windows 7). This is the newest style on Windows Mobile phones, which is very graphically improved. It is specifically built to be more finger friendly, with bigger buttons to press, slightly bigger text, and a new way to flip through settings pages. The menus now usually have gradients; for example, the will fade from light blue at top, to dark blue in middle, and light blue again at the bottom... This adds somewhat of a 3D effect when compared to 6.5. Also, when you use default Windows homescreen in this version, you will see that it has an interface very similar to the Zune main menu screen.. Big words, big buttons, and options to get you to the most common areas.. Email, photos, music, time/date, calendar, etc. These options are presented as text, and you can scroll up and down to choose which category, and left and right to flip through a specific category's options.

Now, the second puzzle piece comes from HTC. To keep this a bit more simple, here is what I understand of it... there are two versions of HTC's interface right now: TouchFlo 3D (Manila 2.1), and Sense (Manila 2.5)

TouchFlo 3D and Sense both have tabs along the bottom. The main difference, though, is that Sense 2.5 adds more features to "cover up" and integrate Windows phone settings into one easy configure tab.. rather then clicking 20 icons to change your phone settings, the settings are now grouped by common features, most with simple on/off switches and sliders. Also, Sense 2.5 has a new People tab, Twitter tab (to see your twitter account right from the main screen), a "Footprints" tab which uses GPS when you take your pictures, and newer, more detailed weather effects on weather tab and homescreen. Another nice feature is that you have shortcuts to 9 of your favorite programs/contacts/bookmarks right from your main homescreen, rather than having to go to your programs tab as seen in 2.1. There is also an option for having an animated background on your homescreen, which I believe is specific to this 2.5.

Now, finally... If you are going for ultimate stability, you will probably want to go with a ROM that uses Windows 6.5 and HTC TouchFlo 2.1. These are the most proven, tested, and stable, and although they are not the newest technology, they very rarely crash and lock up your phone.

If you want the latest and greatest Microsoft and HTC has to offer, you will want to choose a ROM that uses Windows 6.5.3 and HTC Sense 2.5. Both of these pieces (6.5.3 and 2.5) are currently in development by the companies, being tweaked and improved to add more features and stability, and although they have great new technology and many consider them more easy to use, they freeze up your phone more frequently since they are not technically finished and ready for the market.

SO......... Long post, but I hope that clears things up on the differences for those who care to read it! If I mis-explained anything, please feel free to correct me. :)