Worth noting that with Silver on AA you will get:
Flagship Lounge Access (Where available), Soho in JFK. Also if you fly AA from LHR you will get CX lounge preflight
MCE for free on AA with Silver is a good benefit too.
Personally get your 4 eligible flights in SH Y and then do the transatlantic flights on AA.
What will your main Transatlantic routes be btw?