The OLED is by far the best Switch once it's properly undervolted and overclocked. The OLED version also has a better dock with more bandwidth, capable of supporting 4K 60hz or even higher refresh rates at 1080p. This might not seem notable, since nothing really uses it yet, but homebrew is starting to allow Switch games to run at >60fps and an OLED Switch + OLED dock are required to take full advantage of this capability. Just having a V2 Switch isn't enough; it needs to be the OLED model paired with its own dock.
As for undervolting/overclocking, there's no contest. The OLED model runs higher clocks with lower temps and less fan noise in handheld mode, than the v1 can run in completely maxed-out docked mode. The v1 already feels much nicer when overclocked, but the OLED is like having a new console.
Otherwise, the OLED feels sturdier, screen and bezels are much nicer, better speakers, twice the internal storage, etc. all good improvements over the other models.
The improvements are big enough that I actually believe the rumor that the Switch OLED was once meant to be a proper Switch Pro, but Nintendo backed out of spec improvements for some reason or another.
There's a small adjustment, but it's not much different than a softmodded v1. You don't need to use a jig and payload pusher to boot into hekate anymore, you just press the power button and it automatically boots to hekate as long as its files are on the SD card. The only things to get used to are 1) you can no longer coldboot OFW, you have to use hekate's boot entry for that and 2) atmosphere's "reboot to payload" doesn't work on hardmodded consoles, instead you just shut down the console and turn it on again to reboot into hekate.
One final note: if you're interested in undervolting/overclocking, make sure you get the unit with the highest speedos. (They're numbers that basically denote the console's overclocking potential, higher = better). You can just ask the seller to check them in hekate, more instructions here:
https://rentry.co/mariko#checking-speedo-and-ram-type RAM type also matters, try to find a Switch with S-tier or better.