the merseyrail unit isn't a FLIRT, it's part of the METRO family
It's a bit of both - specifically, they appear to have used FLIRT body panels for it. You can see how different the normal METRO ones are by looking at the visually very different Class 555 and Berlin units.
In the end Stadler offer a highly modular single-deck product from a common set of modular components which started out from constructing custom short-run products for Swiss narrow gauge lines, so which brand it comes under is relatively moot.
the GA units where a massive improvement over what came before - whereas the Merseyrail units are, in my view, still an improvement over what came before (they offer level-boarding, for one), the difference isn't as big as at GA
My personal take is that overall they are neither better nor worse overall (the 50x weren't particularly good units either, though they were at least reliable), though they're better and worse in different ways. However, they really haven't got anywhere near their potential due to a raft of absolutely terrible design decisions, from the seating to the PIS to the door layout etc. Unfortunately the views of one person (who did post here for a bit) based on a flawed survey appear to have been allowed to stick, and while some issues raised on the mock-up were resolved they were resolved badly*, presumably because contracts had been signed too early to do a major variation (such as to use an entirely different seat as TfW did with their units following poor feedback on the original one).
* One I raised personally was the protrusion of the headrests, which was "resolved" by making the back cushion a centimetre or two thicker at the top, creating a very unnatural back curve when seated. Metrolink trams appear to use a very similar seat but with no headrest and a different cushion, and it's a lot better.