Different tents are useful in different conditions, so if you want some advice on tents you should first describe the conditions that you will use it under.

That being said, 5lbs seems kind of heavy for a 2 person backpacking tent.

On the differences between Marmot and Kelty. I tend to think their are small differences where Marmot is in general a little better quality. They use a little better materials and have a little tighter quality control. I think Kelty is trying to use a mass produced technique and as a result is able to offer there tents at a great price. All else being equal, I would choose a Marmot over Kelty, but at typical prices you can get these tents for I would probably choose the Kelty. Indeed I own a Kelty tent.

I terms of the higher side walls of the Kelty. It would probably help with blown sand (I doubt rain would be an issue), but it could also reduce ventilation.