I'd have to vote the splinter cell series over the MGS series. Haven't played conviction sorry
MGS4 has a story, Splinter Cell essentially doesn't
MGS4 has better graphics, but Splinter Cell generally looks pretty good
Splinter Cell uses things like close combat moves to make the game differ from a regular FPS
MGS stealth is actually really bad, From what I've played of splinter cell the stealth is only slightly bad (face it, making a realistic stealth game that is actually possible to beat is a task beyond even our best programmers)