Not entirely sure, but I think PComb > Offensive.
If you are able to afford PComb and 20 life scs go and use them. If you are poor and still want to increase smite damage, I think strength charms are the only option, and actually it won't help too much on damage too.
I'm a uber smiter, and what I use is 5x 20 lifescs, anni, ptorch and 4x 15res all gcs. I'm not even going to bother using pcombats because
1. they are ridiculously overpriced
2. they don't increase smite dmg by a lot
3. they are so overpriced
4. they are pretty expensive
5. they are not cheap
6. they cost a lot.
7. they....well you get the point
my v/t uses 9 p combat lifers because well if he didn't then his foh would be useless. In smite vs smite duels with him I have beat people with more life than me because I had the extra damage. So I would say if you can get 30+ life p combats use em. I dont know if the extra damage would be worth loss of life with low lifers or plain combats tho, probably not.
Pcombats provide 15% ed and +3-3 base shield damage, as well as +15% defense.
Offensives only provide 17% ed.
If you can get them, use them. The extra damage and defense is nice.
37x 5 res all, 20 life scs is sort of overkill (up to 225 resist just from charms, as well as gear resist...). You usually wont need more than 80% stacked poison resist (dweb rabies switch or poison necro) or fire resist (facet eschutas+facet viper+2facet circ+4facet jmod = -40% fire res on a sorc tops?). That's where 11/20 single res charms come in.