@Zan I had bad side effects on both Letrozole (6 weeks) and Exemestane (6 months) & then swapped to Arimidex/Anastrozole (4.5 years) and after a few months, I noticed that there were way less side effects - so I hope that happens with your swap too.
I honestly think it doesn't matter WHICH one you try first, that it (the first one) will usually give you the worst side effects & hopefully by swapping, you'll get relief from the side effects xx
From memory, the Onc gave me a 4 week break before starting the new one ....
All the best