I have to confess before we get started. I've only played one Splinter Cell game (the very first one on the GameCube) and STILL playing through my first Metal Gear game (MGS2: Sons of Liberty on XBOX 360 through the HD collection), so I can't comment on my thoughts, but I still ask, which series did you like better?
Metal Gear Solid
MGS2: Sons of Liberty (PS2)
MGS3: Snake Eater (PS2)
MGS: Twin Snakes (GameCube)- A remake of the original PlayStation game
MGS2: Substance (PC, PS2, XBOX)
MGS3: Subsistence (PS2)
Splinter Cell series (all were multiplat on XBOX, PS2, PC*, and GameCube)
*See Double Agent's note below
Splinter Cell: Pandora Tomorrow
Splinter Cell: Chaos Theory
Splinter Cell: Double Agent- Note that this version is different than the seventh generation version (the PC version being the same as the 360/PS3 versions)
Choose your favorite series and perhaps your favorite game in each series!