I used to use Paint Tool Sai - but switched to Manga Studio after I lost my Sai activation code & my computer blew up. I made the switch, as I found Manga Studio better suited my needs & what I was personally looking for from a comic-making program.
For sketching/inking/tone/typesetting, in my opinion, Manga Studio>Sai. The automatic vector line feature with inking on Sai, I found, made me go all OCD and nit-pick things to death - making me waste TONS of time. Just my personal experience with it, though.
But I'd almost say I preferred Paint Tool Sai's coloring & overlays better.
And, while Sais inking vectors made me waste a massive amount of time, Sai did have better quality image scaling when you wanted to re-size your inking (probably due to the very vectors I'd griped about earlier).
I don't know, it's a tough call. It could just be the nostalgia talking - it could just be I'm just not completely used to Manga Studios features yet. But I'd say just use what you're most comfortable using... I mean heck, I've seen artists who make seriously epic drawings using Microsoft Paint of all things... Crazy, right?
Generally speaking, tools have only a small part to do with the quality of your work. But, if you feel like you're having to constantly fight with your tools - then a change is indeed in order. (For example, drawing with a computer mouse... Cringe).
But if you're not battling with your current tools, don't expect a whole lot of improvement if you change to another tool set. Sometimes you'll seem even less skilled on a new program, simply because you're still new at the program. (When I changed to Manga Studio, my art looked like it regressed by 2 years. I caught back up pretty fast after about a month, but that learning curve is there)
If the option is available, go ahead and try other programs out & see whether or not they'd be a helpful addition/replacement for you. (Usually these programs offer a sort of 'trial period' that lasts anywhere between a day and a month, but sometimes not. It's worth exploring.). But the tools you're currently using, (Paint Tool Sai & photoshop), I wouldn't say are a bad tool set - especially to start with. (I can't afford photoshop, so I can't say much about it - aside from 'I wish I could afford photoshop', lol).
There really is no 'best tool' -- only 'what's the best tool for me and my needs?. And that's not really something anyone can answer - it's more something you'll just have to explore & see for yourself.
Best of luck to you and your future comic, and merry Christmas!