AAdvantage Executive will give you an Admirals Club membership, which means you can use the club even if flying another airline.

American Express Platinum will give you access to Admiral Club only when flying on AA, plus some codeshare flights. However, it will also give you access to SkyClub if flying Delta, US Airways club if flying on any airline, plus a Priority Pass Select membership which provides access to Alaska Airlines Boardrooms and hundreds of independent lounges worldwide. Note that UA lounges are specifically excluded from this type of Priority Pass.

American Express Platinum will also reimburse $200 per calendar year in incidental fees, see this thread:

http://www.flyertalk.com/forum/ameri...s-aa-only.html

...and it will reimburse the cost of joining Global Entry or NEXUS:

http://www.flyertalk.com/forum/ameri...efund-usa.html

There are additional rewards from both cards if you intend to use them for spending.