I usually buy PSA, when I can find a good deal, and then resell them in order to finance my raw or SGC Slabbed purchase of the same card.
Occasionally, if I have trouble finding a decent SGC or Raw - I will send PSA Slab in for crossover. Keep in mind that PSA to SGC does NOT always work but works better than most other TPG crossovers.
PSA slabs usually sell better on eBay than other TPGs so if you can find a deal - flipping is usually not a problem. PSA has a HUGE registry following so is usually easier to sell the "Slab" instead of the "Card" to those folks.