We got two of the Champ Elips Padded (green) back in 2008, £30 each from Riversway back then, and they have been brilliant, they are plenty wide enough for us, the XL was huge so no need for the extra for us.
We have two of these, bought about 6/7 years ago and still going strong. DH has an XL version, I have the standard size. I bought some cheap seat pads to go on them, which made them extra comfy and also stops draughts blowing up your back on cooler evenings. They are very comfortable even with out the seat pads. Would definitely recommend them. As for which size, try and sit in one, even if it is the standard size, if you can and then make your mind up from there.