I am planning to buy either of those but I am not sure yet. Thoughts?
Depends on your gaming. I can tell you the pro controller outperforms the joy-cons when focused on traditional gaming feel and originally I was not into the pro controller (thought it was bulky, but ended up pleased after using it). But the joy-cons can still get the job done like with Mario Kart.
If you play mostly alone in person, pro controller is a win: cheaper and just for you.
Joy-cons if you plan to have company over and more than 2 players. Or if children will be playing, since the side-ways joy-con are great for little hands.
I have both a second set and the pro-controller. That's the best of both: you still get your pro-controller but you have enough joy-cons to handle 4 players.
If you want the pro-controller for the D-pad, don't get it. The D-pad is the worst of any Nintendo System and is very bad with diagonals. Go for a joy-con pair for precise D-Pad action, even if it makes things like fighting games painful to play.
But if you're just into the joysticks and big buttons, go for the pro controller as the joystick and buttons feel amazing. Though if you're into a more pricier version you can get the HAYABUSA Arcade Stick which is considered a pro controller and has a high quality joystick.
Ask yourself; do I play games for Joy or to be a Pro.
If you're playing with friends the most understandable and easy to play controller is the better choice.
I don't really see the point of buying a pro controller yet. They recently restocked yellow Joy-Con, and mine are much improved over the set that came with my Switch from late June, though to be honest that Grey set look like they might be refurbished.
People complain about the Pro's D-pad so I assume that will get hammered out eventually, and I'll wait till the word is good. Even so, you don't get a Pro when you're playing portably. I can move from playing with the Joy-Con grip at home to playing portably in Splatoon 2, and my muscle memory has zero complaints.
$10 more, and you have the opportunity to support another two players instead of one. If you know you'll play at home for the most part, and you're interested in a Pro, go for it. I hear no one regrets it.