My experience in reflow ovens...
I was assembling high mix low volume (higher volume was farmed out). All designs were sound (ie DFx). Delegation to upgrade from thru hole to surface mount landed the choice of a 10-12 zone with Nitro. At a cost of over 60K (used) I opted for the Heller 4 zone (thinks its a 5). All the oven folks have this "Engineering" oven. Brand new out of the box was 25K, I spent the remainder on additional ergonomic equip, an RF superMole and controlled atmosphere items that had high dividends. A large issue, not to be underestimated is floor space. The 10+ zoners are huge! I think if you ask the question of which oven is best then you probably need the smaller.
Good luck,
Boston Brad