If you are going to be doing 90% freeriding and 10% park I think the Roundhouse would be ok. It's certainly more of a freeride board. That being said I ride a Roundhouse and love it. Yes it's a bit stiff but that doesn't mean you can't take it into the park.
Also - there are lots of Burton haters on this forum but it really sounds like the Custom is a great all around board and may be the better choice for you.