Definitely take the Hellfire Warlock PRC.

Firstly, it's very powerful. Your damage will be mediocre as a warlock until you hit epic levels, but with the Hellfire class that changes.

Secondly, you can circumvent the downside (CON damage if you use the abilities) by stocking lots of lesser restoration potions (which are cheap) or getting a friendly cleric to cast Death Ward on you, or getting an item that prevents ability drain. The latter two solutions can be regarded as cheats, but eh. It's not like warlocks are overpowered even with this bonus damage.
As for levels lost: the Hellfire 'lock stacks with your normal warlock level for everything that matters, and it's only 3 levels anyway. In Mask of the Betrayer you can reach level 30, so you lose hardly anything by taking the prestige class. (Specifically, you lose 1 bonus feat and 5 points of Energy Resistance. To which we respond with a hearty "meh.")

Thirdly, it lets you summon some cool demons that will be very strong when you get them. Funnily enough you can even summon some of the plot-involved demons from the OC. Including Neeshka. That's in Storm of Zehir, though. I wonder what will happen if you try it in the OC.

Fourthy, finally, and most importantly: it makes your character look very cool.