Having played all of the games in each series, I feel that they both have their strengths and weaknesses, but I have more issue with the Splinter Cell series.
When it started off it was very strict and not doing something "the way the game wanted you to do it" would more often than not lead to failure. Add to that the immense amount of bugs in each game of the series (falling through floors, disappearing save games), it just hasn't endeared to me as much as Metal Gear Solid has, even though that is quite strict in it's own way.
Looking at Conviction, as pretty as it is on an old engine, I don't know what it's trying to be. The stealth is there, but now it seems that you could just shoot your way through a level if you liked. I know that is exactly what happens in MGS4, but that was never what the Splinter Cell series was about for me.
Will see when the reviews come out.
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