The trouble with chargeing a 6S lipo with significant capacity with a 50watt charger like the Thunder AC-6 is time. For a 6S, charge current will be limited to 50watts/25.2v = ~2amps. This reduced charge current (which is typical for pretty much any charger when your charge exceeds the charger's output power) will increase charge times unless capacity is small and the 1C rate is about what the current is limited to. If you were tyrying to charge a 5000mah 6S, teh lower charge current would take over twice a long to complete a charge. It your 6S lipos is 2000 or less, it will take what most lipos take to charge, about an hour depending on how discharged it is to start with.

A dual charger is nice if you need that capability, but most don't and use a siongle channel type charger. The resale on the DUO will drop when the new SuperDUO hits the market just like I've seen my 0610iNet chargers value drop as the 0720iNet came out and the new 1420iNet due out sometime in July. Resale is a minor factor in buying a charger, to me its all about functionality and the ability of the charger to meet your charging needs.

You're kind of stuck in that you want a cheap charger, but need to charge high cell counts. Those needs don't go together well, any charger, especially a cheap charger toward at bottom of the price range is going to be a compromise and the cheaper it is, the bigger the compromise.

If you would specify what capacity (mah) your 6S lipos are and if you want to charger at higher than 1C (capacity(mah)/1000 = 1C) charge rates, it would help narrow down options. Do you want AC/DC or does it matter?