I suspect, if you have patience and wait, the Big-5 sale will be repeated. IINM, Big-5 has frequent sales on Remington 870s and Mossberg 500s. Just wait until they offer the configuration you're after. While waiting, try to get some trigger time with both the R-870 and M-500. Some ranges have them as loaner or rental guns.
IMHO, the prime difference between the M-500 and the R-870 is the location of the safety. Since I don't rely on safeties, I prefer the R-870 since its safety is less obtrusive. Also, if you want to use a stock with a detached pistol grip, the M-500's safety is awkward to operate.