I like the tonality and grain of Delta 400 more, but I also started shooting right after TMax and Delta films came out, so it's just the film I started with. It seems a lot of people feel the exact opposite. Most people try out a number of different films and stick with what they like, others like to keep experimenting.
If you do have any plans to scan your negatives, supposedly Delta scans better than HP5.
90% of my B&W photos are taken on Delta 400, exposed at ISO 500 and developed in DDX--this used to be what Ilford recommended but I don't think it appears on the film box instructions anymore (the recommendation to expose and develop at ISO 500, that is). I also shoot Delta 3200 exposed at 1600 but developed at 3200 (tonality seems better), and FP4. I like Delta 100, but after I learned I could develop FP4 in the same batch with the Delta 400 and 3200 negatives, the convenience won out. I haven't done much testing with the 100 speed films because I don't shoot 100 that often--usually only on really sunny days, but sometimes I still shoot with the Delta 400 and use a 3x ND filter.