It depends on which type of 2D you want to produce :
- cartoon-like sprites : Illustrator (vector based)
- realistic/stylish handmade sprites : Photoshop/Gimp (pixel based)
- cartoon : as you search to work with flattened colors delimited by distinct zones, you will never have a better control on it than with vectors. Vectors let you control absolutely every single curve point you want, to a nearly infinite precision.
Here is some artwork compilation I made for my current game, with Illustrator :
- realistic/stylish handmade sprites : All power to your fingers. As a real drawing style speaks easily with your own hand, not your mouse, it's better to use a pen tablet. Mouse interface would be way too clunky in a direct handdraw. You pen tablet unleashes tons of awesome painting styles, like the size of your brush, its pressure, its graphic style, etc. It lets you produce tons of different layerwork in no time.
Well this one is not so realistic, but the shadow treatment could hardly have been done with Illustrator in the same working time :
You can even use both supports, which is handful for 3D textures : Vector for the base pattern, and tablet for the shadows/style details.
This is what I used for my chars, and it came really handy, making the textures far more precise :
Now the price can be a choice breaker :
- Vectors : all you need is Illustrator (no other free software had equal power), which is ~1000$/850€
- Pixels : you will need a good pen tablet, with a good pen sensitivity. Lower (acceptable) models start at 400$/300€, higher models (LCD screen, which can't be beaten for precision imho) are about 1500$/1200€ to 2000$/1800€. But here you can use a free pixel editor instead of photoshop.
Personally, I didn't hesitated to purchase those licenses + a LCD pen tablet, and I don't regret it at all. It's quite an investment I agree, but trust me the wonderful things you can do with this stuff is far more valuable than the price.
So in the end, for the same price, you can access the 2 techniques. Your choice.