Because of the amp circuitry on the 1500 series as compared to the 300 series I would expect the 300 series to sound "better" (maybe more body and immediacy/presence of sound), but this is NOT to say that the 1500 series wouldn't sound good, and the 1500 is much more compact and light with enhanced conveniences (like the jack location as Herm mentioned).
I've always been intrigued by them for this reason. GREAT portable mixer for 4 and 8-track mixing. Full-featured. Typical Tascam design of value-engineering without cutting important construction, quality and feature facets.
If you have one available to you locally for a good price I figure you can't go wrong.