Same seats. If you get the A319 FCO service, pretty much the same aircraft too. The 737 service will be marked out only by (usually) being more tatty...and having a horrid blue rinse any OAP would be proud of in the bogs!

Broadly the same type of catering offerings for the equivalent time of day, if maybe not exactly the same dishes...and if you like Port, you're better off to/from LGW as they're the only ones that carry it ^ thanks to Alpha doing the LGW catering and Gate Gormless doing LHR's

I'd have no qualms over taking the LGW service(s) instead - in fact, I am doing just that to FCO for a short weekend trip in a few weeks. I mostly enjoy my LGW CE trips, they're normally very good.

LGW will have less crew on board (minimum crew - 3, unless there's a very big CE cabin at least one direction) but IME they're excellent and hard-working and you'll still get great service in CE...just sit as far forward as you can (and hope you don't get one of the bad Pursers - which exist in both LHR and LGW - who insist on starting service at the back against the rules ).