Skip and Jana are simple to learn, but hard to master characters who can really blast through stages!! Both of them look absolutely adorable and cool respectively, and each bring a lot to the table. They also have really amazing sprites and animation, and their easter eggs are small but get a smile out of me every time.
Skip is really fun to control given his collect-a-thon style and you'll be skipping across levels really easily with his dives and monitor abilities! He's more hard to master than Jana but easier to learn IMO, as his dives will *smack* into walls- but if you get his dives down perfect, you can gain really high speeds and heights that can easily speed through even Aerial Garden. I really adore his monitor crafting, and how mechanical parts spill out of busted badniks and monitors in the zone already. The shields you get from crafting and each zone buff Skip really well, making combat easier as well as platforming. That's my only issue with Skip really... I haven't gotten used to dealing with enemies as him as you can only beat enemies while falling or by diving, and I often miss my dives in Simple controls, which I'm more used to (and also makes him play closely to his inspirations). Bosses aren't as hard thankfully, given their large hitboxes!
Also, the flashlight is a really nice tool even though you don't see it used often. Hopefully levels with dark, secret-filled areas get made so he can take advantage!
Jana on the other hand, is a dream of mine realized. She brings the fast pace and insane mobility of Zero to SRB2 almost flawlessly!! Jana has not only a really fun dash which can be jumped out of for mobility, but the wall jump shenanigans she can pull off make hard levels quiver in fear, and with her saber, put them out of that miserable fear! Her slashes are quick and rarely dampen the pace unlike Amy and her hammer, and the charge slash can get tricky airborne enemies!
Jana's wall detection for wall jumps is also nigh perfect, though at times I can wall jump only to cling to it a second time without height, but given a second jump I hop off perfectly. (Also, a part of me deep down wants her jacket to flap more in the wind when she moves, blame Zero's successor Gunvolt for that. Jana doesn't *really* need it though as she already looks so cool, haha!)
TL;DR, both Skip and Jana bring fun abilities that you'll be figuring out cool tricks with every time you play, while looking awesome doing so.