The APUGGERS above have given an excellent summation of the differences between HP5+ and DELTA Professional 400:
The fundamental is that HP5+ is indeed a conventional type film which is very forgiving and has excellent latitude and an excellent all round film.
DELTA Professional 400 is a very modern film using 'CCG' or controlled crystal growth technology, this gives very fine grain negatives ( for the relative speed) and some cases may not have the latitude and is less forgiving in relation to poor exposure control or processing regime.
You can check out our website www.ILFORDPhoto.com and also if you PM me your home address I will post you some literature etc that will give much more detail. Good Luck.
Simon. ILFORD hoto / HARMAN technology Limited :