Personally I'd rather two 3-day weeks on site per month (with paid travel & accommodation). This is easier for balancing family etc as you only need to be away two nights for each trip. But it really would depend on the person and their circumstances.
If you're looking at whether you can attract people outside the HQ city, I think the two factors are frequency and length of in-office time. More than a few days is tough for many to be away from home, and more than a couple trips a month starts to be a lot of travel. I say this as someone who has regularly flown to a different city for work.
If you're just talking about attracting people who are a 90 minute drive away vs 45 min (for example), 1 or 2 days per week in office each week might be easier to swing.
