He was correct in that the vented model does dry more quickly and I'm sure you will get a lot of responses on the merits and demerits of the combo unit.
Our 2000 Beaver had a vented model and our 2002 Marquis has a ventless model necessitated by location. Both work fine and DW will not be without one regardless of a bit more drying time.
Stacked units are nice but you do lose a lot of storage so that is a consideration. For us, the combo works just fine and it's not worthwhile to consider changing.
IMHO, don't worry about it until you've been using the combo for a year or so than evaluate you options.