i just obtained 3 kegs, 1 straight, 2 bulge. If you have the option, using the bulge would be better for liquids and not the mash. So either the BK or HLT would be fine. I have read that you do sacrifice efficiency when your MLT is anything but a straight walled cylinder. my guess being that the bulge pools more wort around the perimeter of the grain and less filtration through the middle. I dont know if there is any proven science behind it, but it makes sense, and if it can be easily avoided, why not.
I still am not happy with the look of 2 bulge and 1 straight. I think on monday i'm gonna make a few stops at some local stores and see if they'd be willing to swap out. Doubtful but who knows.
i started a thread similar to this a week ago...check it out: