Female to a female would probably be easiest, but it depends also on the attitude of the male if you had fallen in love with a male. Generally in the cat hierarchy males are down near the bottom, and females rank highest. If you fell in love with a REALLY laid-back male, who would seem to just roll over and be submissive to the female, then that'd probably be okay. But otherwise, another female would be your best bet, since there's little need to battle over positioning in the hierarchy - in comparison to unspayed females which rank at the very top, spayed females rank very, very low in the hierarchy. And that is mutually understood by any spayed females in the picture.
It also depends, of course, on the personality of your current cat. If she is very dominant even with you guys(I've read that cats who don't cover their feces in the litter box are often the dominant ones), then you'd probably want to adopt a kitten, and in that case, whether it was male or female really wouldn't matter. But if she's more laid-back, then another spayed female around the same age as she would probably be okay.
I can only give advice based on the few experiences I've had with introducing cats. My fiance and I have two cats right now; one isn't spayed, and about 13 years old, and has been dominant all her life to the point that even when she was a kitten, she did not hesitate in attacking a much heavier, adult cat, because he hissed at her human. She latched onto his rump while he went screaming through the house, lol. Her room-mate is my cat, who is spayed, and about 4 years old now. There's still tension between them from time to time because the dominant one isn't fixed, and this feels it necessary to assert her dominance from time to time, but they get along more or less.
My parents recently got a friend for their current cat, who they adopted from a friend of my mom's who could no longer care for her, and she is a spayed, 11 year old female, who was predominantly an outside cat. An outside cat, not in the boondocks, mind. She had been outside every day during the summer since she was 2 years old when she was adopted, but kept inside during the winter. And she has loads of street-smarts. But she was obsessed with marking her territory by clawing everywhere even before my parents got another cat. The cat they adopted was a 6 year old spayed female, and VERY active and playful. Usually they avoid one another. Sometimes Missy, the 11 year old one will attempt to chase Snickers, the 6 year old one, and she'll run. Sometimes when Missy is walking by where Snickers is laying, Missy will hiss at her. But they *never* fought. Not even when first introduced. When they were first introduced Missy just ran and hid. So Missy seems to be dominantly insecure, which....really isn't a good way of being dominant. My fiance's cat is a very confident alpha, and all the cats(and dogs, lol) that she meets respect that.
In hindsight, your overall best best would probably be either a male or female kitten, or a 1 year female, which, at 1 year old, isn't technically an adult. It just skips the rambunctious kitten stage
Because introducing another adult cat into the house of a pre-existing cat, can sometimes prove difficult to get them to the "buddy-buddy" stage, since that house is the pre-existing cat's territory, and she may or may not like the idea of another adult waltzing into her territory all because of your orders, lol.
So...there's my input. Again, it all depends on the personality of both cats, if they're both laid back, it's a good fit, if they're both dominant, probably not a good bet. If one's a rambunctious male coming into the picture .....may not be a good bet, lol. Simply because males, that I've read, try and work their way up the ladder, and if there's a female present, the female generally beats them back down the ladder, lol. It depends on exactly what kind of relationship you're looking for, too. If you want your kitties to cuddle with one another, again, adopting a kitten might be best, but if you don't mind the possibility of your cats ignoring one another, and possibly not being seen around together, or at least not seen in the same room for consecutive days, than another female the same age would probably work.
Again..limited input from limited experience, sooo....