It IS possible to both pillar bed and glass-bed a stock at the same time. However, why would you want to do so ? No reason to do this - so, it's generally considered a "one or the other" proposition.
As to which is better - neither. Done correctly, both will produce equal results. Pillar bedding does help somewhat to prevent stock compression over time. This is not a really big issue in most cases, however.
Pillar bedding IS more difficult to get right than conventional glass bedding. More alteration (irreversible because large holes for the pillars must be drilled) to the stock must be done. Mess it up and the stock might be junk. This method also produces more localized stress on a stock. With wooden stocks, in SOME circumstances (such as weak areas of lower density wood), pillar bedding can lead to cracking. For these reasons, I prefer conventional bedding.
I would NOT recommend doing both on the same stock. I would also respectfully disagree with 4runnerman about trying pillar bedding first. As I said, this method is more difficult to get right and can be problematical if one has little or no experience with it. Once the stock is drilled for the pillars, there is no going back (unless filling mis-drilled holes with epoxy or plugs is acceptable to you). Conventional bedding is easier and takes advantage of the inlet areas already existing. As these inlet areas are designed to spread the recoil forces throughout the structure, rather than only through narrow localized portions (as with pillar bedding), conventional bedding tends to put less localized stress on a stock. So, there is a lower chance of inducing cracks, etc.
Often, if the action has a stout, relatively large recoil lug already and if the fit of the action to the stock is good (already), glass bedding can often be limited to the recoil lug pocket (on modern actions) - with acceptable results. (This does not apply to older style actions, such as Mausers, which require more extensive bedding.)
As I said, either type of bedding will produce good results, if done correctly. Oh and I would tend to agree with 4runnerman in that, if the rifle is shooting really well now, it may not need bedding. Most rifles can benefit from some work in this area, but, as I alluded, it is possible to get good results with an abbreviated bedding job, depending on the type of action and the (existing) fit in the stock. It is always best to start small and work up - and this is true of bedding projects as well.