I don't have any experience with the polymer beads but know that they are 2-3 times more expensive than aluminum oxide and don't last near as long. 50lb of aluminum oxide will run you about $20 - $25 now so it's not a cheap prospect.
Walnut shells are good for hard plastic and durable plastic parts like kick panels. They're also good for cleaning but not necessarily stripping soft aluminum parts that have a nice finish-like a manifold or a PMD timing chain cover. For rough cast aluminum you might as well use glass beads.
The problem you mentioned with stripper is exactly why you don't want to get it into any seams or crevices. I've never been a big fan of strippers because they're so messy and I don't believe that there is much to be gained in time savings.
