The problem there is that the cheap crap is usually crap.
The BT SA isn't horrible. It's very similar to the PCS USP that was around awhile back in the day. It's a stickfed system, meaning there is no additional magazines to purchase, which will keep your operating costs down. The SA is a little "space-gun" ish, but I find lighter than both the TPX and the Tib series.
I also find as a former store owner and tech, maintaining and repairing an SA was much easier, cheaper, and quicker than both the TPX and Tib series.
I find that mag fed does give you a bit more realism, especially for scenario events.
Also, reloading a stick fed can be a bit of a b*tch compared to dumping mags.
It really all boils down to what you plan to use it for.
If you're going to play in scenarios with it only, with big events, and lots of people, I'd get the SA. You can throw a hopper and a in-air source to it to be able to get you get more versatility.
However, if you're going to play in a "CQB style" playing facility, where realism matters, and speed is key... the TPX > Tiberius for me personally. Mags are cheaper, more inexpensive upgrades like barrel systems. (TPX is A5 threaded, as oppose to Tiberius is proprietary)
Either way, you can scoop up anything "recent" on the used forums here for around $100-150 pretty easily... so, see if you can try out both, either, or 1 before making a purchase from a local player or friend at your field. This will tell you what fit would be best.