I'm currently kegging 2.5-3 gallons of each batch, bottling the balance. So, I'm using both CO2 and sugar to carbonate brews. I would advise using gas to carbonate kegs, since it will result in far less sediment/trub in the keg. I would advise reading the
sticky by Bobby_M in this section for carbonating in keg. I had to trace down leaks in my gas lines last night, eliminating them all, so I decided to try his method. I should know by Saturday/Sunday if it works as advertised.
I will say that everything I've read about carbonating kegs with sugar indicates that you use less than if you were to use bottles. I would advise using one of the better online calculators to get the actual amount to use per volume, temperature and desired CO2 volumes. OR just bite the bullet and get the hardware to carbonate with CO2.
I'm planning on kegging all of the batch currently in primary (an English IPA that will get dry hopped for a week first). I look forward to the shortend bottling session that will result from kegging. Since I have more than one gas source, and regulator, I'm even thinking about carbonating one keg inside the fridge, and the other (at least partially) outside of it. I'm also thinking about getting another regulator (for the normal tanks, since the other one is for paintball bottles) to help make this easier. I do believe you'll get more consistent results if you use CO2 with a keg than sugar to carbonate.
I am planning on comparing (along with a fellow home brewer) at least one batch that I bottled and kegged. This way I can see what the difference is (if there is any at all)... I'm hoping that they are so close you cannot tell a difference between the two. Or that the carbonation is better in the kegged part.