I'm flying from Boston to London in March, and I'm going through a booking website so I can play around with different airlines without it effecting the price. I've flown Delta more times than I can count, and I've never really had any issues, but from what I've heard from friends and what I see online, Virgin airlines looks much more modern, inviting, comfortable, etc.?
Can anyone give me some comparisons from personal experience?
I know some of Virgins smaller planes might not be so great, just as some of Delta's aren't, but I'm flying overseas so I'll definitely be on a 747.

Virgin and Delta code-share on BOS-LHR so look carefully to see who will actually be doing the flying.

Neither Delta nor Virgin use 747s on BOS-LHR. Delta uses a 767 and Virgin an A340.

How does Virgin's A340 compare to a 767? Do you have a preference as far as either airline?

Are you flying economy or business (upper class on VS)?

If it is business class, VS is better than DL for comfort, upper class cabin and seats are really great and I prefer the A340 over a 767, but service/meals is better on DL.

I prefer VS business class to Delta business class any day.

Have not done economy in both to compare, sorry. In biz -- VS gives you jammies, a nice little bed, attractive cabin, decent food. Delta is fine too but VS was a bit more enjoyable, and the lounge at SFO (from where I departed) was much nicer.

i'll be flying economy, thanks for the comment on meals. i have always been satisfied with deltas in flight services

In economy, I'd go Delta. The 767 is the best long-haul economy experience out there.