This is a constant point of discussion, sometimes, one might almost say, contention.
1:50, 1:48, 1:72 etc are generally referred to as "scales" - proportionate representations of real-life. So, 1:50 is fifty times smaller than a real life object.
28mm is not a "scale", It is a randomly chosen height measurement for miniatures which allows a combination of relatively (sometimes stunningly, particularly when enhanced by the phenomenally outstanding painting seen on forums such as LAF) good detail with relatively compact size for tabletop "skirmish" (for wont of a better word) size gaming. Its basically a compromise between those two criteria - a model you can recognise with some personality and wanting to be able to fit a reasonable few of them on a 3 x 3 ('ish) board. It also tends to be fairly liberal in its interpretation of body mass and extremities in order to be able to give more visible identifiers for players and more area for details and expression for painters. Hence the term "Heroic Scale" within "28mm" and other "scales". - this tread is, politely, known as "scale creep", although, given its not a "scale" in the first place. . .. . . . All of this of course means that selecting/buying miniatures that might remotely match each other, or your own hopes, expectations and aspirations, is in itself "challenging"
To add to that bit, most companies who make 28mm compatible items (such as vehicles) seem to base their scale at 1:56.
In my mind (other minds are available), 1:56 is pushing the small side for 28mm minis, particularly given the scale creep mentioned above. Personally, I tend towards the 1:50 scale you refer to for most of the vehicles I use with "28mm" figs.
BUT, and its a big BUT, 1:50 figures will, in my experience, TOWER OVER (so, be several mm taller than) "28mm" figs - but at the end of the day, it is, as it always should be, down to whatever lights your fire.
Enjoy, and I'll shut up now and apologise for the diatribe.
ATB, G-Donk