most rabbits are happier if they have a friend of the same species
if I were you, I'd make sure to learn how to sex rabbits before going back to consider the other one - if they're definitely the same gender (confirm this for yourself, don't trust the pet store folks!), you can keep them together until their first fight and then get them fixed, wait for hormones to die down and then re-bond them. if they're an opposite sex pair, because you can't be sure of their exact ages and because some bunnies are early bloomers, you would want to keep them separate (but near each other) from the get-go to make sure there's no chance of an "oops" litter.
the benefits of getting a second bunny now:
#1 you JUST got the first bunny so pretty much your whole house is neutral territory. not that it matters because...
#2 babies will bond with anyone
#3 if they bond as babies and you get them apart before they can get fight much after the hormones kick in, it makes reintroducing them later much easier as long as they're always close enough to see/smell each other without being so close that they can fight through the bars
#4 if you're lucky, they might be able to get fixed before they even get hormonal, at least w/females (with males, they generally get neutered once the testicles descend which I believe is also when the hormones start... females can be fixed once they're 4-6 mos depending on the vet and their size and puberty may or may not have kicked in yet.
I think an opposite-sex pair is risky with babies, though, because you have to separate them so young to ensure no chance of a pregnancy that they won't really have time to establish a sufficient bond and once they're fixed adults, you just have to cross your fingers that they get along.
the benefits of waiting until after your bunny is fixed and the hormones are totally gone:
#1 no worries about that first fight; no hassle of having to house them separately for what could be as much as 2-3 mos depending on how quickly after they get hormonal your vet is willing to fix them and how long after that it takes for hormones to subside.
#2 you can find a shelter that allows bunny dating (where you bring your bunny in to meet potential matches) and let your bunny pick their own friend, increasing the chances of a successful bond. most that allow dating also allow you to return/exchange the second bunny if the bond just won't work. this means there's no chance of being stuck housing two bunnies separately on a permanent basis if they don't get along as adults.
if you want a M/F pairing, I would definitely go the second route of waiting for your bunny to be a fixed, non-hormonal adult and then finding a shelter bunn.
I currently own two females who are almost 5 mos old but haven't had any fights yet *knocks on wood* and are due to be spayed in 7 days, so it looks like the 2 babies thing is going to work out very well for me without ever having to separate/re-bond... however, that's certainly not always the case and there is an element of chance involved in getting two rabbits as babies.
