|Personally, I prefer the AFs, for two reasons. The main one was that when my tubby DS#1 wore Rikkis, if I didn't have the velcro adjusted perfectly, it would rub his stomach. The second is that as he got over 30 lbs, it got harder to fit a L Sandys with liner into the L Rikki at night. DH also didn't like the variability of velcro--he just likes knowing which snaps to use, done!
That being said, it's completely personal preference, as they are both stellar covers.
I've never used the XL in either, as DS #1 PTed just as he was pushing the limits on the diapers and covers, and DS#2 is tiny. I would guess that the XL Rikki would fit a bit tidier than the XL AF at 30 lbs, esp. over an OS rather than Sandys or ToddleEase (they are bulkier).
ME will let you mix up covers for the cover special; I'd guess that they would let you get both different sizes and different styles if you called them (and they have great customer service). If you got a mix, you could try them on over a fresh diaper (and maybe a fresh night-sized diaper) to see what you think, and then exchange for the kind you want to get. That's a bit of extra postage, but would let you make a truly informed decision. I'd call them to double-check that that would be an okay thing to do, but I would think that as long as the covers are just tried on, not used or washed, they would be fine with it.