I prefer NGC. PCGS slabbed coins "seem" to have a premium attached to them over and above the cost of NGC coins. I also think that NGC slabs look better.
As far as the service itself, it is definitely PCGS. There are many, many coins where a PCGS slab willl command a substantial price premium in the open market over an NGC slab at the same grade because it is believed that PCGS' grading standards are more strict - and I believe that they actually are more strict (too strict?) in several areas. On the other hand I cannot think of one area where I think NGC is generally considered to be more strict than PCGS. The vast majority of my 100-or-so slabbed cloins are in PCGS holders. As far as the holders, I think the ICG holders with the intercept shield cannot be beat. Actually, I think ICG grading is very good, too, but for whatever reasons (probably marketing) they have just not become as widely accepted.
In addition to having my paper money graded by PCGS, i use them for coins to they are tuff to Beat when it comes to resale of the same item and grade i would pick them first
NGC is considered more stricter by many on Franklin's Full Bell Lines (FBL) as one example. The Full Bell Line designation that NGC applies is when both the top and bottom bell lines are struck fully and without interruption. The Full Bell Line designation that PCGS applies is when the lower sets of bell lines are complete and some slight disturbance of the top lines lines would be acceptable. I use either TPG's depending on what coin is being graded.
oh no!! Not this question again. Different TPG's for different coins/series. Stick to the top four and you will be ok.