I've been watching the Next Generation for a while but on my dad's recommendation I started SNW and I love it. I'm getting through it really quickly and thought about jumping straight to TOS after.
Is it going to be really jarring or do the two sync quite nicely? Do the characters feel a bit flat after all the exploration they get in SNW?
That's a good call, with a caveat.
I started watching TOS after seeing the 2009 movie and was quite, er, taken aback with the change in pace lol. I was still able to handle old Trek because I watch a lot of older TV, but I've tried to show certain classic episodes to my husband and the pacing frustrates him even when he's otherwise really enjoying the story.
The pacing is pretty bad on a lot of episodes. Especially how long they like to just hold on a shot of the crew waiting for something to happen. Sometimes seems like hours and hours of this.