Neither would work, they don't have dedicated cards. Both would completely and utterly fail at Left 4 Dead. Run TF2 at lowest settings, 640x480, at 20 FPS. . .
Newegg.com - ASUS M51 Series M51Ta-X1 NoteBook AMD Turion 64 X2 RM-70(2.00GHz) 15.4" Wide XGA 3GB Memory 250GB HDD 5400rpm DVD Super Multi ATI Mobility Radeon HD 3650 - Laptops / Notebooks
The dedicated card makes all the difference in the world. This thing would tackle Left 4 Dead and TF2 nicely, both games are great, I own them both. I have a laptop that's got just a little bit of a slower graphics card (also ASUS brand, same line) other than the graphics card, they've got the same hardware. I plug in 2 external monitors to it, each at 1680x1050. I run Left 4 Dead on one of them at the full resolution (1680x1050) with high settings, at 30-40 FPS. Left 4 Dead is way more demanding than TF2. This laptop should kick the **** out of both games.
It's just $100 more than one of your options, and the difference is epic. Newegg is very reliable by the way. Oh and if you wanted this before Christmas, newegg's cheapest shipping method guarantees it delivered on the 24th. If you place the order by 11:59 PST 12/18. Newegg is very reliable.
Don't get something without the dedicated card. Hope this was a help.