I am also a long time D&D player from way back, since 1978. I have been playing RM since 1983. So am going to second Chosen's opinion and IMHO, HARP has D&D beat hands down!
H&S offers a quick based, but deadly, combat alternative to HARP. If you want a little bit less deadly combat system for HARP, that also takes damage reduction from armor into account, then you could always use Martial Law. Martial Law combat is not as quick flowing as Hack & Slash though, and it required more rolls.
I like the more varied damage results in ML myself, even if it takes a little longer to run combat.
The spell system for HARP is what really sold me.
HARP has D&D beat four ways to Sunday in this regard. HARP has more spell using variants and options than D&D, plus the ability to scale spells. Furthermore, HARP spell casters are more playable at lower levels than in D&D since the system is based on spell points versus x-number of spells per day.
Finally, HARP allows you to create vertually any type of character you want, unlike D&D's cookie cutter templates.
Just my 2 cents.