I've just bred my first litter. All the pups were born healthy and beautiful, for which I am very thankful! I planned to keep a girl (and there is one) but find myself drawn to one of the boys and now wondering if I can part with him! The girl is gorgeous too, I just don't have the same connection with her. Could I keep them both?! I don't really think that is an option.

keep the one you love. But do bear in mind the practicalities of keeping a dog and a bitch.

Do not keep two puppies.  See links. 

https://youtu.be/LSEno_YxhMk 

https://youtu.be/jfZ5n-N1sJk

http://www.whole-dog-journal.com/issues/13_1/features/Problems-Adopting-Two-Puppies-At-Once_16190-1.html

http://blog.betternaturedogtraining.com/2013/07/18/littermate-syndrome/

Why not keep both for now? And spend more time assessing them, and then you can sell one on at a later date, when you have made your pick.

Run them both on and then make your choice. I bet you keep the dog

Yes, as others have suggested run both on....you havnt stated what age they are now, for most breeds the best age to make you decision is 5-6 weeks.
If you do run them both on, I bet you wont want to sell either but you have to remember, if you are limited on numbers, you need to be sensible or you wont be able to add in a few years, when you might want to.

Yes, that is a huge consideration!

Yes, if I decide to have another litter in a few years it will be a big consideration adding to my pack.

It is really hard to adequately socialise two puppies - independently of each other.  They won't be able to be taken out together, they will need individual outings for socialisation, separate crates to prevent excessive bonding to each other... restricted playtimes with each other whilst wanting to reach each other.

I'm a behaviourist and trainer and I see the fallout of littermate syndrome all the time.  Please take it seriously and read the links I posted above.  If you want each puppy to reach their potential, give them their own home from 8 weeks where they will get 1-2-1 individual attention and focus - they deserve it.

What was the reason you wanted to keep a bitch ?
Has anything changed with your plans on that front  ?
How would keeping just the boy change you plans ?
Would keeping a boy with the existing dogs work ok as far as you can see ?
As for keeping two I know people who have done it successfully but its very hard especially as you have other dogs as well.

Hah - nine times out of ten, we keep two from our litters!   In fact on one occasion, we had a super litter, one of our best, of 9 puppies.   I'd already decided to keep two of the 4 bitches but then the owner of the sire came to see the litter and said I'd be nuts to let the best male go.   So we kept 3 and brought them all back to the UK with us plus mum, uncle and two others.

Provided you have the accommodation, there's nothing wrong in deciding to keep more than one puppy but the decision has to be yours.   Why not run on both, for now and make your final decision later on?

Taking on two puppies at the same time may not be for the novice/ inexperienced, but I'd lay odds most experienced breeders do exactly this, with none of the difficulties suggested here

I was keeping a bitch so that I had the choice to have another litter from her in the future. It isn't a given, it just gave me a possible option, all being well with her.