At +5, I'd pick them over Cleric 15 any time. At 16, unless I had a specific build I really wanted to try out, I'd also pick it over Cleric 16 without any real hesitation.

A LA of +7 would mean they would get 9th level spells by 20, yes? I don't thinly it's a bad trade at all.

But then again, the Playground is deathly afraid of any LA whatsoever. +6 is indeed the lowest I would go, with a strong argument for +7 IMO.

The slew of SLAs and Special abilities more than make up for the lack of prestige classes.