The NWN2 remakes of both games are in fact very well balanced and preserve not only the feel but the combat balance despite the toolset / rules difference.
Frankly they are both excellent true renditions lovingly created and are worthy of play (buying the originals especially at the low expense is a great idea especially for nostalgia and the actual community mods that enhance those games in and of themselves; true D&D junkies like me want copies of all the games)
In IWD (v2 I believe) the author provided a difficulty slider (?) I kept v1 and still play it ; but the boss fight wasted me 3 times before I won at regular levels even with a specific weapon. I am no "expert tactician" but I am no noob either especially as it applies to D&D games.
1) The IWD (v2?) still lacks the Heart of Winter and Trials of the Luremaster expansion components - so the original is still needed for these scenerios - again another reason to buy the originals.
2) BG lacks Shadows of Ahm expansion (although one is in development); SoA is the creme de la creme of BG. It is probably the reason the BG series maintains its reputation as the greatest of all RPGs. Again another reason to buy the originals.
In my opinion the difficulty is not based "so much" on the rule set although the differences can handicap an unfamiliar player.
It is more based on the fact that BG and IWD are low level lower power games for much of the early stages. 1 Orc can in fact waste half a party if it gets a few lucky roles , and or the player is not prepared, and or you wander off the road a little too far. 1 group of Goblins with bow and arrows can cause significant damage and death as well. Add breakable iron swords, and a lack of inexpensive "full armor" and you have D&D much as it was in Pen and Paper (original and 2nd edition).
This contrasts with both the NWN games where the loot / money scripting allows a very inexperienced protagonist to upgrade to +1 and +2 gear - in addition to "full armor" and "damage suppressing belts" fairly quickly --- and mobs or goblins become overly easy and routine.
I suppose this is by conscious design choices ; therefore I will not critic it - just note the difference.
Recommendation:
1) Play the original versions through. The stories are very much worth the time investment
2) Play any and all iterations in NWN1 and NWN2 ; the updated graphics / familiarity of the toolset / and even repeating the story is worth it for the story's sake. It brings a certain sense of deja vu ~ like visiting an old friend.
Off the top of my head:
1) NWN1 Heart of Winter by Glenn Pudny (?) (very good author) was a nice NWN1 version of that scenerio
2) The NWN2 remakes - both as noted.