MH Wii was a repackaged PS2 game.

Resolution and framerate have an impact on the level of graphics you can display. Lower resolutions and lower framerates means that you are putting less stress to the hardware and therefore you have room to use better graphics, more poligons, better and more demanding effects...

But on the 3DS case this is irrelevant in most cases, because the system has to take into account a limited power suply and very limited heat generation due to its portable nature.

Anyway... Monster Hunter 3:

MHTri: 480p, generally better textures (rocks, vegetation...) and effects (the water is a very obvious one), locked 30fps

MH3U (3DS): 240p, lower quality textures and effects, better shadows, unlocked framrate (30-45fps)

So... The Wii is flexing double the resolution, overall better graphics and a stable framerate, while the 3DS runs at half the resolution, it mostly looks worse with some exceptions like shadows and it sports an erratic framerate.

I don't know, it don't see the 3DS coming out on top here. Not even close.

3DS. The games were simply far, far better for the most part, including franchises like Zelda, Fire Emblem, Pokemon, the Dragon Quests (you could play every Dragon Quest from IV on on 3DS), as well as originals like Bravely Default and 7th Dragon 3 Code VST. I also give the 3DS Fire Emblems credit for giving rise to Three Houses.

The Vita became a Nihon Falcom machine for me, with the first two Cold Steel games, Ys: Memories of Celceta, and Tokyo Xanadu. All of these games have migrated to the PS4, and the series continued on Switch as well. It also had a decent backlog of PSP and PS1 games, such as Breath of Fire 3 which is kind of hard to find otherwise,, but I mostly played those on the Vita TV. The Vita was a lot of wasted potential, but honestly there's nothing it really does that the Switch doesn't do better, whereas the 3DS still stands on its own.

Vodacixi beat me to it as I was offline (funnily enough playing Monster Hunter) but 3 Ultimate on 3DS had cutbacks compared to Wii like worse textures and water effects as well as the lower resolution.

3DS. Amazing portable! It has a huge library full of true gems and many of those are exclusives

For me Vita, since I love to play narrative games on the go.

• point&click classics like Day of the Tentacle, Full Throttle, Grim Fandango
• new point&click adventures like Broken Sword 5, Broken Age, Demetrios, Guard Duty, Machinarium
• Telltale Games series like The Walking Dead Season 1 + 2, The Wolf Among Us
• adventure/VN hybrids like Danganronpa 1 - 3, Zero Escape 1 -3, Corpse Party 1 - 3
• visual novel series like Clannad, Code:Realize, Chaos;Child, Grisaia, Muv-Luv, Spirit Hunter, Steins;Gate
• JRPG/VN hybrids like Persona 4 Golden

The 3DS also had Zero Escape 2 + 3, but I prefered the Vita versions.

Besides the awesome Ace Attorney and Layton series, not many story-focused games were on 3DS. I'm glad that this changed with the Switch.

3DS. Nintendo actually made games for it.

MH3 was the best looking MH on console for a while.

3DS screen is 240p only, handheld always has low resolution screen compared to console, but i'm comparing 3ds ver on 3ds screen to Wii ver on TV screen here. It wasn't better but also not worse, almost equal.

But on 3ds screen, those things almost didn't exist, and definitely not "significant downgrades".

It was the ONLY console MH post PS2 until Wii U rofl. It wasn't that special looking. Talking about a lot of small flat contained areas with load screens. Xenoblade is far far far more demanding than that. Massive open areas with a lot on-screen at once. Textures in the New 3DS version were much worse. New 3DS could not handle the game in full.

I've seen MH3U on 3DS, the downgraded water and textures are still noticeable and significant. 

3DS is simply graphically less capable than the Wii. You can talk about the extent to which the smaller screen hides this, which is subjective, but at the end of the day the 3DS just can't push as many pixels or details as the Wii can.