I've never heard of these games but I want them so bad now!
If you want the most girl/girl stuff, make sure to end up with Sugar as the summon partner in the first game. Pratty (that's the female MC's official name)/Sugar's interactions are often pretty funny. (Basically, Sugar hits on the MC equally, regardless of your gender.) Otherwise choose any of them; it's a great game with any summon.
On that note, yes, I forgot to mention your summon ally. The "Summon Night" name is meaningful, you have a summon ally who you talk to and such. They can help you in battle a bit too. "Swordcraft Story" refers to that you make weapons this series. The main characters in all three games are both warrior and blacksmith.
As far as the characters go, the male and female MCs in the first game are in front (center and left of course), surrounded by the various summon options. You'll only have one protagonist and one summon partner in each game, of course; you choose your summon through some questions at the beginning.
The box for the second game has the female MC in the center. The male MC is the one in the lower left. The rest are summons and a character from the story. And yeah, now that I think about it, SNSS2 did have a good story too; it's not quite as great as the first game overall, but it does have a decent plot. It definitely has better graphics too; I like the first games' floating city setting more, but they did upgrade the visuals. The third game looks similar to the second.
The main Summon Night series was a Japan-only strategy ('SRPG') game series, but none of those were released in English, only those two and a DS action-RPG. In Japan only there are also a couple of traditional RPG Summon Night games on the DS, as well as remakes of the two PS1 Summon Night strategy games. Unfortunately the DS game we did get, Summon Night: Twin Age, isn't nearly as good as the GBA games. It's alright, a decent top-view hack and slash game, but it isn't nearly as great as the Swordcraft Story series unfortunately. It's not quite as funny or well-written, either. Feels more generic. I still liked it, but not as much. Oh, one Flight-Plan SRPG did get an English-language release, Eternal Poison (PS2); it's not a part of this series, though, that's a separate game. I haven't played it.