In my experience, 2.5mg letro is stronger than 25mg of exemestane. Exemestane is also more likely to be faked than letro due to cost of the raws.
Letro is great for someone who really needs a lot of e2 protection (someone who already has gyno). If you're just looking for on-cycle protection/insurance and you're not sensitive to e2, go with exemestane.
I also recommend exemestane in pct.
It's always good to have some letro just in case things get out of hand, though.