I have an E320 in brown and a Ducane three burner (Weber product) in SS. The E320 is a whole lot easier to keep clean on the outside than the SS Ducane. The E310 and the S310 will cook the same and both will probably last the same amount of time. My choice would be the E310 just for the easier maintenance.
But yeah that SS sure is pretty.
Why not get the EP-310? For just $50 more than the E-310 you get SS flavorizer bars and grates. More than worth it IMO. You can't get these at the Big Box Stores like Lowes and HD. They're available through Weber Authorized Dealers like Ace Hardware, etc.
Me, I'd get the S-310 (is the side burner the only difference?).
I'd also buy at Home Depot & get the PCI (porcelain-coated cast iron) grates.
If you go to the post office and grab or ask for a change of address form, they'll give you an envelope that contains a 10% off coupon to HD or lowes that either of them will honor.
I recently sold my house and the buyers wanted my S310 which was 2 years old. So I had to buy a new one and I got the E310. The stainless was too much trouble to keep clean and shiny and the E was about $75 cheaper. Both will cook well and meet your needs.
Ray
I would spend the extra money and get the EP-310 to get the SS flavorizer bars, rather than on the S-310 SS model. I didn't want the SS model, which is why I chose the EP-330 over the S-330.
The EP series is a great deal compared to the regular E series. You get stainless flavorizers and grates, plus that sweet lookin' "Special Edition" emblem. Go see your local Ace Hardware.
My EP-330 in copper arrived yesterday afternoon. Thanks for the input on which grill and why. Side burner was a must for lighting charcoal chimney for wsm. She is a beauty.