FP4 and Delta 400 are the 35mm b&w films I regularly buy at retail, both are easy to use and provide great results. (I do buy other films but typically its only when I find a bargain.)
FP4 is truly a joy to use, beautiful and easy to work with as long as you have enough light.
If I had to pick just one of these films though, it would be Delta 400 without question or hesitation; the 1-2/3 stops in speed difference and it's ability to cope with a larger range EI choices simply makes it much easier to shoot.
I do try to meter well and shoot my Delta 400 accurately at 400-500 but I will happily shoot Delta 400 anywhere from EI 50 to EI 3200 if needed. Delta 400's flexibility/latitude means if I'm in a hurry I can just use aperture priority or if using a manual camera I can just skip readjusting exposure and shoot knowing I'll get a workable negative either way.