Well, how much are you spending locally? Remember, local means tax, internet means (most of the time) shipping and no tax.
Most often, the S38's are sold in pairs (about $500?). The S310's are $279.99 Each plus shipping with coupon.
S310's from Amazon
Use code
AMZN-ELECTRNC for $50 off a $200+ purchase, buy each separately using 2 accounts, of course.
Overall, you're not really saving THAT much money going with the S38's.
Don't forget, you're going to NEED a subwoofer. It's not as critical that you match the subwoofer to the rest of the speakers as it is matching the speakers to each other. Does that make sense?