Really? You must be just considering the IRS vs solid axle. That's a rather minor difference. Did you know a stock 3rd gen can flexes more than a stock 2nd gen? Having owned two 2nd Gen trucks (one on 35s and one on 33s) I can tell you the 3rd gen is as capable if not more so than the 2nd Gen. My rear diff has 15" of ground clearance. My front skids have over 12" of ground clearance (and that's with only 1/2" of suspension lift). I'm still running the factory diffs and the traction control is nothing short of incredible. It is absolutely the best system out there. I do have 4.90 gears and ARBs sitting in the garage for it. These trucks are way, way more capable than people in the US think they are. My biggest complaint after driving many miles off road is the t-case gears are horrible. Seriously considering buying the 3.15 gears.
I've driven my Montero stock through the TransAmerica Trail. Then took it to southern Oregon and drove the McGrew Trail. Then on a 2,300 mile expedition through Montana & Wyoming 4wheeling into different remote lakes every night (several that were straight rock crawling). Went up into Washington (Mt Rainier) this weekend and drove the Naches Trail and some technical trails in Evan's Creek. In total, I've driven just short of 10,000 miles since purchasing the truck in April. And it was down for 30 days while rebuilding the top of the engine. And it's not my daily driver, just weekend adventure rig.
The Montero fits way better on trails than the 100 series (it's a tank). It has better ground clearance, better traction control system, stronger differentials, better visibility, better gas mileage, more interior space. Plus some nice things like: spare tire on the door (no swing out needed), aux gas tanks, huge storage compartment (with 3rd row seat removed), fantastic 5 speed transmission. The driving/seating position is perfect which is mandatory if you drive 12+ hrs.
I've built many trucks: 83 Hilux (on 36" tires, 22re swap, dual tcases), 92 Montero (33s, 5" lift, skids), 95 Montero SR (35s, 5" lift), 86 4runner (SAS, 35s, gears), 92 Isuzu pickup (ARBs, Gears, 33s, RTT, armor), 13 Toyota Tacoma (expedition build over $20k parts). And I'm having a blast with this 03 Montero.
My point is simple. You really need to seriously consider and compare the 100 series to the 3rd Gen.