Round 1: Graphics
Metal Gear without a doubt
Round 2: Gameplay.
Metal Gear, I'm a sucker for stealth
Round 3: Fun Factor.
Metal Gear never gets old. Then again, I played RE4 over a hundred times, and thats not an exaggeration or including Adas story (around 60 times normal and over 40 times on professional). I've likely put more time into RE4 than I have all the MGS games combined (ground zeroes alone got a good 25 hours out of me, so I've played the hell out of MGS too), as awesome as they are. Thats not even counting all my play throughs on 360 or PC. Every time RE4 is ported to a new console, I buy it and replay it until I unlock everything. Co-op in the other games also adds a ton of replayability. It would be fair to give this to RE since RE4 has pulled me since day one, but I'll give this one a tie I suppose. I'll never get bored of either.
Round 4: Sales.
MGS4 sold 5.67 million copies worldwide
Resident Evil 4 sold 7.4 million copies (GC, Wii, and PS2) and this was before it came out on next gen systems and PC.
This goes to Resident Evil, though it is important to note that MGS4 made all its sales on one console which is pretty impressive. RE4 had the benefit of being constantly remastered and ported around.
Round 5: Critical Acclaim.
RE4 is one of the most critically acclaimed titles of all time and was considered revolutionary. You'll rarely find a top 10 video games list without RE4 in the top 3 somewhere. In its time, it scored almost perfect reviews and even higher than MGS4 (though the same as MGS2).
I'll give this to RE.
Round 6: Story/Stories.
Metal Gear in quite the stomp
Round 7: Upcoming Titles.
Once again, Metal Gear in quite the stomp
Round 8: Current Titles.
MGS has more consistently good titles. RE has been in a sharp decline since 4.
Which is the better series?
Honestly, as a whole, MGS. Great protagonists, great villains, great story (mostly), great gameplay.
RE has great protagonists, but the majority of them have been brought back as two dimensional shadows of what they were. Chris and Leon got the worst of this. If they were aged and built on in a better way, they'd be on Snakes level but they both peaked too early.
Despite this, RE4 is my all time favorite game. Silent Hill 2 comes in at second place for me (while also being my all time favorite series). MGS4 comes in at 3rd though it will be kicked out when MGS5 comes. Ground Zeroes already confirmed that.