It depends, you know? Multi Theft Auto was always better because there were many new addons, many new plugins, objects etc. etc. that were never presented in SAMP. So obviously, the quality is much higher because MTA has a lot to show. Though frankly there are a lot of more players preferring SAMP over MTA, most likely because of the quantity of servers and type of servers.
Generally, MTA is focused on racing and deathmatching. So people that enjoy racing just head to MTA, because it has much more options in races than in SAMP. Though people that enjoy team deathmatch, or perhaps role playing, head to SAMP - because there's a larger quantity of role playing servers on SAMP and team deathmatchers.
Simple as that. :>