I recently flew ORD-LHR-BOM on AA/BA and the T3-T4 transit literally took over two hours of standing in security lines. The problem with transit in DEL is that you'll likely need to stay overnight due to the arrival time of the AA flight.

While the LHR transit option is inconvenient, I think it's the most reliable because BA was pulling people out of line who risked missing their flights.

Keep in mind that LHR enforces the 1 carry on bag rule VERY aggressively.