Both.
I know what doesnt help but you have to look at it both ways and narrow down the reasons for "yes, this one because..." and "no, not that one because..." That should help you decided a little better. Do you know the temperaments of them yet???
So, my vote is for both, like I said.
Yes to the
girl because...
-Her coloring and markings are absolutely beautiful!
-You don't have a female yet so it gives you a chance to have both sex's
-She will probably be a tad bit smaller cause shes a girl (?)
-All her things can be cute and pink like her collar and leash (at least that's what id do lol)
-Your dogs could be girlfriend and boyfriend which would look so adorable together!!
-She would match your male

-Give you a chance to come up with a cutie girl name since you already have a boy named
-You'd have a chance to have one of each sex, a boy and girl (which in my opinion would be fun!)
No to the
girl because...
-She looks alike like your male
-Your male is not fixed, therefore it could cause a problem (unless you fixed her,him or both of them)
-You would have to buy different colored things to match a girl, since you have a boy and boy colors (if your gender color coordinated like me

)
-You might end up with a accidental litter
-You would have to deal with her something into heat which in some cases call for doggy diapers
-Spaying is more expensive than neutering
Yes to the
boy because...
-His colors are stunning
-You could tell them apart easier since they don't look alike
-Its cheaper to neuter than to spay
-You have no chance of any "oops" litters being born
-You can call them "Your boy's!"
-Your dog would have a brother to get along with
-They could bond better together
-He wouldn't go into heat so you wouldn't have to deal with what comes along with that
-You can use the boy name you liked but didn't get to use when you where naming your other dog, to name this new puppy
No to the
boy because...
-You already have a boy
-They might not get along as well as your dog with a female would
-One or the other might try to dominate the other
-You wouldn't have the chance to experience a girl doggy
-His coloring isn't as pretty as the girl (im my opinion)
-You might have to deal with him marking since he is a male
-You have a boy and a girl would be something a bit different
-They two males together could be a bit overpowering
-There is a chance that if they don't get along by them trying to be dominant or trying to fight, you would have to get both fixed with if you had a girl, you would only have to get just one of them fixed.
So, my thought on this would be to narrow it down like I did

Good luck!