Sai is GREAT for colouring and lineart and has VERY EASY TO USE vector/pen tools (it's different from how photoshop does it though, so you'll need to play around to get the hang of it).
UNFORTUNATELY, Sai will be missing some things that you are used to from photoshop: gradient tool, shape tools, text/typing tool, dodge/burn (it kind of has a similar thing, but it's not the same I don't think).
It's great for getting the drawing and art done, but it has very basic or none of the colour and photo editing options you might be used to from Photoshop (no fancy filters, no clone or stamp tool, etc.).
I use SAI for most of my artwork personally, drawing directly in the program without needing to scan in a sketch, but I still use Photoshop to finish editing if I need to make colour adjustments or even add in my website/copyright/watermarks.
If you have the money, downloading Paint Shop Pro would be a good alternative to Photoshop.
You can also try out Fire Alpaca, which has a lot of features Paint TOOL SAI is missing and is free (it just has a pop up ad when you start it and maybe the brush isn't AS nice as SAI): http://firealpaca.com/en
Or Krita also looks pretty good and has a lot of the features you might like too and is also free--it seems to be constantly improving and I'm pretty impressed with how it looks anyway: http://krita.org/features