Main difference is the grain.
HP5+ has the "traditional grain" which usually produces higher edge contrast and sometimes higher apparent sharpness. I usually say that hp5+ has a certain "bite" to it that lends itself to edgier compositions. Compare it to Kodak's tri x.
The Delta film had delta grain (probably epitaxial or similar to Kodak's tabular grain) and has very smooth tonality and high level of detail. It is usually described in more dreamy terms.... subtle smooth, creamy, gentle... Compare it to Kodak's tmax 400, which is arguably superior.
In terms of overall tonality, you can make one behave like the other. The grain and edge contrast is the main difference and that is quite individual.