I can see the advice here is to avoid 90-minute T5-T3 connections. That said...
In early September, I'll be flying CPT-LHR-ORD. I have the choice of a 90-minute connection to AA's maybe more-lightly-loaded 7:45 am LHR-ORD, or a 6-hour connection to the 12:15 pm departure (coming from J on the CPT-LHR flight to Y on LHR-ORD on a mileage ticket). Right now, I took the longer connection, figure I'll give myself a lounge tour, get some tea at the Harrod's store, read the British tabloids, etc.
Would you roll the dice on a 90-minute early-morning T5-T3 transfer?