If you want to experience true to fact legend of a game HW1 is, play it original. If you have any technical considerations why not, and don't really care about marginal flavour sh!t you might be losing, there is the remake.
I can't comment on actual details as I have never played the remakes, but from the read about, most of the difference stems from fact that HW1 remake is an attempt to port that game's content onto (remake of?) HW2 engine. This may not necessarily be a very good idea, because each game have some quite unique aspects and peculiarities. Part of that can be, and I suspect is, lost.
One major difference that might or might not be relevant here (I don't know) as it have more to do with level design than anything else, is: while in HW1 you usually beat a map, then gather lose resources (or not), rebuild your fleet, make research etc, then jump away when feel ready, often to situation you have to resolve immediately, in HW2 level wraps up as soon objectives are reached auto-gathering all the resources there could be, then you may, or may not get a lull period at the start of the next map, with AI only reacting once you prove your presence. But you get such slow starts in the HW1 as well, and overall the HW1 I remember as slower peaced by the feel of it. As to difficulty, both games occasionally trow you in few incredibly brutal situations regardless of the scaling factors, but those may add (or remove) few of those. The ability to 'over-build' your fleet in HW1 (by capturing anything that can be captured, mostly) above what difficulty scaling can compensate may make the endgame more easy than it probably is supposed to be; what unbound scaling Hw2 style can do to it, is hard to say, but it may not be pretty.
It might be influenced by countless factors (including highly unrelevant, unrelated and pesonal), but imho, HW1 campaign was much more enjoyable and solid, and just better done than the second one.
Post edited March 04, 2018 by Enneagon