Ilford offers HP5 Plus and Delta 400 Professional. There is no "HP5 Plus Delta Professional". Both are high quality films, but they do have different characteristics. The tonal scale and grain structure of the two are different; if you print them on the same paper the pictures will look different. In general, HP5 Plus is a more forgiving film, meaning that it is easier to make negatives with it under a wide range of shooting conditions that will print well on a wide range of papers. Delta 400 is also a fine film, but is somewhat more demanding of technique. There's nothing fundamentally wrong with Delta 400, but I would not have recommended it for a beginning photographer. Also, if you are used to HP5 Plus, you should stay with it until you have gained more experience, so that you don't get confused by different results from the two films. HP5 Plus is very widely available, so if one dealer happens to be out of stock it should not be difficult to find it elsewhere.
Hope you're enjoying your photographic explorations - good luck!