If the cost is not an issue:


and if it is, then one is free and the other one isn't!

Illustrator is a commercial program for commercial/market needs.
It has better support with printing -colour modes, saving to eps-, better with kerning (?), layer modes (?), and has vector based gradients.

Inkscape is open source, has the ability to add your own extensions.
It is based on the svg specification.

Which is better for you depends on what you are using it for.

I'm using inkscape because I see adobe's suite elitist.
In my opinion you can drive perfectly in a ford as well as in a lamborghini,
but scratching the paint on the second one is totally lame.
And there are people who actually think they can drive well just because they could afford the expenses.