As far as I'm concerned, it's pretty close, and it really comes down to the individual ride.
Generally, I tend to have more comfortable rides on boomerangs. It's very rare for one to beat me to a pulp, whereas I've ridden my fair share of SLCs that were utterly vile. On the other hand, a good SLC is a much better ride experience than a boomerang.
Given the choice between a relatively smooth SLC and a boomerang, I'd take the SLC any day. However, if the SLC is starting to get rough, I'll go with the boomerang. It'll hurt less.