You are going to have to decide what aspects important regarding features and layout. I have a 2006 Pioneer 180ck, nearly identical to the Mallard. We are a family of 5 and we love the layout, including the bunk (I sleep under it near the wall, doesn't bug me but YMMV). That bunk means at night all 3 kids get their own bed, during travel, I have a toddler folding bed rail installed, and all our suit cases travel up their, at night they go under the dinette and in the bath tub. During the day the kids like to lounge on the couch and watch some TV. The Salem layout would not work nearly as well for our family of 5.
We camp without hook ups a lot, and cook a lot, so the 2 burner stove-top with no gas oven would be a deal breaker. The convection oven would be worthless. When cooking breakfast I usually have all 3 burners plus the oven going!
Our Pioneer has nearly twice the holding tank capacity. 50/40/40 fresh/grey/black vs 34/25/25. It also has ducted A/C and ducted heat, and 2x30lb propane tanks vs 1x20, and room for 2 GC2 batteries. There were a few changes in 2007 so I am not positive all these stayed the same.
Our Pioneer has been very sold, only a few minor repairs that I was able to handle myself. Worst was a brake wire inside 2 of the hubs was too long and rubbed through causing a ground out during braking. Once I found the problem the fix was easy.
However, the Pioneer / Mallard is also 1000lbs heavier than the Salem, about 4000 before water and stuff, vs 3000 before water and stuff. You mentioned V6, but not what vehicle. The Pioneer was towed reasonably well by our 04 V8 Explorer, the V6 variant would have really struggled. However, many current V6 have as much or more power as my old Explorer did. The engine is not as important as cargo capacity, with the family aboard, you need to know how much cargo capacity is left for the tongue weight. A V6 Highlander is not going to tow the Mallard well.
Biggest difference is the Salem is a single axle not a tandum, and with a dry weight of 3000 lbs, and GVWR of 3900, you don't get much capacity for water or stuff, only 900lbs and that single axle will be MAXED out. 1760lbs for load range C 14" tires x2 = 3520lbs, so axle is likely a 3500lb axle. MFR is figuring (praying) on at least 400 of the 3900lbs will be on the tongue. This is a typical problem with single axle trailers
The tandum axles on the mallard are not even close to being maxed with a 3800lb dry and 5600lb GVWR. 1760lbs for same tires = 7040 lbs, even if the axles are only 3000lbs, the trailer running gear is capable of supporting it's entire weight not depending on the tongue weight resting on the tow vehicle.
Notice the label on the mallard, it has over 1300lbs cargo capacity AFTER propane and full water tank! I don't ever worry about how much stuff my family of 5 packs. The Mallard frame is also heavier (from what I can see in the pictures) 6" vs 4"
I will say that is a good price on the new unit, not so good on the used one, there may be room for negotiation on the used one.
Good luck in your search!
