This is a good question, though I wager it comes down to how you attach the plates to your flak vest. For example, Legacy should be more flexible, unless you make the plates all attach to each other, effectively making one single plate. Then standard is more flexible. Personally, if you do legacy I recomend searching out the V2 version, which Ohl'd Vart created for his son, I'm getting ready to use them right now.
Xaviorbat is correct...it depends on how you mount them. On the plates I designed, my son made his a single unit by attaching the lames to each other....I have seen people use a webbing suspension system between the plates, too, so they would have some flexibility.
All in all, it's an "aesthetic" thing...the V.1 Legacy make you look like a tank, and the V.2 (mine) are tapered to give the illusion of a thinner waist....one reason why I made them, is because I'm on the 'heavy' side myself, and wanted a chest design that would help detract from my wide qualities....something a Modern set won't do very well.
The link to all the versions are in my sig below...it's a tutorial page.
Rob