The smaller diameter rollers will provide more resistance.
I have the performance rollers too. They have a 3.5" diameter drum.
If you're looking for a workout, I'd get something with a smaller diameter as these don't offer much resistance.
Yeah, I got some new knock-off ones for about $100. But beware, some (if not all) will vibrate/rumble while riding on them, so if you don't live on ground level, you might want to wait till the downstairs neighbors leave. Or use them outside (in which case you might end up just riding around the block a few times instead :P).