I'm a big fan of Mission Falls cotton. I haven't worked with Rowan Tweed and can't comment on a direct comparison, but it's easier on the hands than a lot of the cottons I've worked with. Generally speaking, I find 100% cottons hard to work with for long periods of time 'cuz of hand cramping but when blended with bamboo, rayon, or even acrylic, it gets easier to manipulate. (In the affordable cotton-blend category, I'd recommend KnitPicks Shine.)

Going up a needle size often helps with the hurts-my-hands thing I get with cotton, so if you like the fabric you're getting with a larger needle, it may help.

Your best bet might be be to see if you can find a ball at one of your local yarn shops and swatch with it. You probably won't get the same dyelot as a Webs order, but for a mitred afghan, dyelot's not a huge deal.