First, although no-one would use an A321XLR on LHR-LAX, they couldn't as that sector is over 4,700nm (it's 4,741nm).
Second, there's much whining about the thing only having a single aisle. But a regular 777 has seats in 3-4-3 anyway, so it's not like your getting any less space: the 777 has 4 middle seats per row, the A321 has 2.
Key point for me would be journey time. If I can fly DEN-EDI non-stop I'll take it over faffing around in London, Amsterdam or New York.
(when aircraft type is a factor in my decision process, it's the cabin altitude that determines, not often the seat layout, although JAL's 787s win big on both...)