Difference between the westone UM and the numbered series (true fit) are rather varied, although they are ultimately very similar. For example, westone UM3X is not the same as a westone 3. Generally the UM line is catered towards musicians, stage performers, and use in studios, while the numbered truefit lines are for audiophiles and personal and mobile usage as well as personal listening enjoyment. There is nothing wrong with using a UM for personal listening as well, which I personally do, but you might not find the sound emphasis to your liking.
As far as the emphasis layout and audio signature goes, the difference is that the UM series focuses more on mid range sound and has a more focused sound stage, while the Numbered series places greater weight on highs and lows (especially bass, UM bass is a common quibble with those who use it for consumer listening purposes) as well as having a larger feel of a sound stage. Generally the sound stage refers to roughly how possible it is to pick out specific and individual sound sources in a virtual sound field in your mind.
In general, the UM series are known for having a transparent casing, while the numbered ones have a more conventional and sleek exterior. The westones 1,2,3s look more commercial whereas for the UM you get the feeling that they are earphones less for the common consumer. In matters of aesthetics of course, it does boils down to personal choice. UMs also seem to fit better into the ear canal, but once again this depends on the kind of year you have, as well as the ear tips you intend to use.
As for similarities, there are quite a few. The number will refer to the number of drivers that the earphone will have. For example, a westone one will have 1 driver, a UM1 will have 1 driver. Also the number often happens to be (perhaps by a quirky coincidence) the first digit of the price tag. Like, a Westone 3 comes at $399. They are both high quality phone lines, and prices start from the steep end of 100+. They both have funky cables, that are braided together like a rope.
If you are into bass it might perhaps be a better idea to stay away from the UM series and go for the numbered ones.
If earphones are not to your liking then you might want to consider headsets or headphones.