As above, disregard the Fenix. It's a very cheaply made marker and you'll find yourself hating it.
With the Reflex Rail, it's a good buy. It's not great on paint or efficiency, and requires a bit of maintenance. If you don't mind cracking it open after every weekend's worth of play, wiping down & re-lubing the firing assembly, they'll shoot well, for a long time. Take the time to learn the manual and understand how it works, as well as put decent (Duracell Ultra/Energiser/Energy Paintball) batteries in it, you should never have an issue. Remember that on Rails, 90% of problems can be solved with a new battery.
The Mini - well, if it fits you from an ergonomic standpoint, and it works, the likelihood is that it will continue to work. However. If it goes wrong, that's a whole other story. They're a bit of a pain in the arse to service & maintain, but they're marginally more efficient and a bit softer on paint than the Rail. A word of warning on the older (G1, G2 and occasionally G3) Mini's is to watch out for the electronics randomly going haywire, the HPRs not functioning correctly and the trigger running away. Suspect build quality was also an issue in the G1's.
(For example, I was on a team with Mini's. Over the course of the day, one started to fire at 22bps for no apparent reason, and wouldn't stop until the air was turned off, the other one had a battery fall out mid game and one just plain stopped working. This was all with 1st Generation Mini's)
For most people, I'd recommend the Rail over the Mini. However, if you're a younger teenager, the Rail will feel a bit uncomfortable in the hands for a couple years.
