I've had a quick look at the only review of the Antec 300 that's out atm
Antec Three Hundred Gaming Case Review
It looks like a not bad for a budget case, it's not as large as the 900 and lacks the drive cages.
Apart from the bottom psu and a 140mm top fan it looks like a pritty basic budget case.
It looks large enough inside
The CM690 is a very nice case, looks good, plenty of room, good tool free stuff
Only down side is you cannot fit IDE hard drives in it's drive caddy system, so the only way you can put IDE hard drives in it is to get some adaptor rails and fit them into the 5.25" bays
So if you've not got any IDE hard drives it's a great case, if you still have IDE drives look for something else.
The Sharkoon rebel 12 would be a nother one to look at, not hideiously expensive and very large inside
Computer hardware and software at amazing prices, available online from Scan Computers UK
Although you will need to get fans for it so the price will go up if you don't have any.