I expect that the level difference (26 v 36) is a significant factor in determining where the sweet spot is. I did some calculations the other day using data that I've found on the web, or researched, and it's fairily clear that (for minions) at levels 1-10 it's faster to go after even con opponents than it is to go after +1's, and that at level 15 it's usually faster to go after +1's than even cons. This is, not directly affected by AT but is affected by accuracy - so blasters, especially dev blasters with targetting drone, and scrappers will probably want to switch earlier than tanks or controllers with unslotted attacks.
There are, of course, mitigating factors such as the ability to actually handle +1's, potential higher efficiency on +0's (AoE anyone) and, potentially, extra defensive cost associated with fighting tougher opponents.
P.S. Regarding Methodology:
I collected HP data for MOB's of level 1 through 20, and assumed that a character can attack at an approximately constant rate. That means that to determine relative XP/minute, it's sufficient to calculate
(XP per MOb * chance to hit * damage factor)/(MOb HP)
The HP information was collected using a client during issue 3. The XP per MOb information is from a PDF file available on the web, and the to hit and damage factors are based on accsprd.xls.
