Yes, welcome to EnWorld. Now, on to the question... (my answers are based more on the numbers and abilities aspect. If your game is more into interaction, it doesn't matter as much.)
Either way, I suggest picking one of the classes to be your focus, or main class (since Binder 10/Warlock 10 is not a good combo, whether you do more combat or RP).
If you want Binder to be your main class (most levels), I would only suggest Warlock 4 (though, if you really want, 6 can give you some nice flight ability). Warlock 4 would give you the UMD boost, 2d6 ranged touch, and some least invocations. Spider Climb is usually useful, as is Devil's Sight (nothing like seeing perfectly in the dark, when you do dungeon crawls). With Binder, you may also want to take the Prestige Class "Knight of the Sacred Seal", if one of the vestiges becomes your favorite. It's a very nice class, and you don't lose out on much from Binder itself.
If you want Warlock to be your main class, Binder 3 (particularly with Improved Binding feat), 4 or 5 are good stopping points, in my opinion. If you take Improved Binding, it will let you bind 3rd level vestiges early and you can get some nice little boosts with those (Paimon, for example, will give you that nice +4 Dex). With this style, consider carefully how you want your character to fight... Do you want a hit-and-run style character (Fell Flight + Otiax)? Or maybe a ranged combatant (Leraje + Eldritch Chain/Spear + Spider Climb/Fell Flight)? I don't suggest front-liner, since your Attack Bonus won't be great, and you have lower HP. As for Prestige Classes, there aren't any good ones in Complete Arcane for a Warlock, but if you have Fiendish Codex 2, you can make use of the Hellfire Warlock, and with Binder, use Naberius. It is a very nice combo, allowing you some good boosts, at a cost that gets fixed next round.
If your campaign focuses more on roleplay, you would naturally have a different selection of abilities. The Beguiling Influence invocation combined with the Naberius vestige and some ranks in Diplomacy makes you a wonderful face-man for the group. It really depends on how your group/campaign plays, and what you want to play...