The reason that question is important is that adjustments to film development, away from normal, are only important in relation to your printing process.
If you are targeting a specific paper grade, yes it makes a real difference, if not; then if you really want a good answer we need to know exactly why you would want to adjust the film's contrast.
For example, I use VC paper and prefer a normal contrast rate negative regardless of the measured scene contrast, if I need to adjust contrast I do it by adjusting the paper contrast, if I need more shadow or highlight detail, I dodge or burn.
In my world VC paper is the norm, changing my film contrast rate gets me no advantage, in fact it generally makes printing harder for me. In Ansel Adam's world, fixed grade papers were the norm, adjusting film contrast made printing to his fixed grade paper easier.
If you are having a lab scan it, normal film development is a great place to start. If your working with a highly skilled lab and they tell you they can do a better job if you change, then change. I'd bet though that normal will work just fine for almost any lab.
XTol will work fine with HP5 and is a great choice since you have it and are already familiar with it. No need to buy anything else.
Do you shoot your TMax films at 100 and 400 respectively? Do you develop them per Kodak's instructions or ... ? I ask because you can transfer you prefs from these films to HP5, if you like to shoot TMax 400 at 320, do the same as a starting point for HP5. Same idea with developing.