Well, let's see.
I'm biased towards the 3DS. I've had a 3DS since launch, and put well over 1000 hours into it (New Leaf alone is my most-played game of all-time with 600 hours played). But I finally got a Vita earlier this year.
3DS gets the edge in: First-party games. number of third-party exclusives, lower price, DS backwards compatibility, non-propriety memory cards
PS Vita gets the edge in: Technical specs, third-party games that I actually want to play, two analog sticks
I think both are underrated. A lot of people see 3DS as B-tier and Vita as C-tier. I would put 3DS as A-tier, and Vita as B-tier. I have to overall pick the 3DS for Nintendo games like Animal Crossing: New Leaf and Fire Emblem: Awakening.
The future is the Vita for me though as I haven't had it for long. My 3DS still works but has a damaged hinge. Honestly, I might have to get a used 2DS or New 2DS XL if I ever plan long-term to use my 3DS again.
Lifetime Sales Predictions
Switch: 144 million (was 73, then 96, then 113 million, then 125 million)
PS5: 105 million Xbox Series S/X: 60 million
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima