Depends what you call old and new monsters, it might be that first, second and third generation would be better?
Based on my 3-400 ish miles a week in all weathers
1st gen up to 2000ish (600, 750, 900).. My 2000/2001 750 would be that. Fueling is by carbs otherwise same as same as really. The motor feels a bit snappier than the IE I had, handling is pretty much the same. Fuel economy maybe slightly better than the later IE.
No problem reliability wise other than odds related to a broken wire (fixed in 5 before I scooted off to work one morning) but hey it had experienced 10 years of flexing so really not so bad.
2nd gen post 2000/2001 (620, 750, 900).. My 750IE would have been that. Fueling was injection, never had any problems with it apart from the rubbish clock system. Handling not much changed over the previous version.
If it wasn't for my experience with the clocks I'd say no problems reliability wise.
Parts costs and availaibility not any notivable difference
3rd gen would be anything from around about now ish I guess or when the engine sizes moved on from the old 600/620, 750, 900 formula or they become less classic monster design, dumpy trellis etc.
Oh and those water cooled things get a category of their own
should we call it an 'interesting diversion' ??
LOL