I haven't dealt with UL in some years now, but a few years back they could be very strict in their interpretations. I'm not specifically saying that's a 'bad' thing, I guess. They simply, at that time, had tough auditors whose interpretations I often disagreed with as too strict.
Now days ISO isn't such a big deal as it was 10 years ago. Interpretation issues were much more significant and were much more common than today.
I am NOT recommending against UL. Just stating my experience.
On the other hand, registrars are popping up everywhere. It's only US$20K to become one. Considering travel and such, I look for local registrars. Normally they're a bit more expensive on the auditor hours but with no travel expenses are cheaper.
As I tell my clients, I will give them several 'recommendations' but they should do the interviewing and make the decision.
Two spreadsheets for registrar comparison are in the ISO Implementation Guide Files package and the Cove Premium Access files area. If I'm not mistaken, the spreadsheets are also in the 'Cove Members' area as well. I think they're also in the Free files directory as pdf files.