US Air first class? or AA's 737 business class
 

Dont know wich one to pick for my return.

US Air first class? or AA's 737 business class
 

I assume this is an international segment? Depending on the route, AA business class on a 737 is the same as US Air first class on their equivalent (A319 - A320). It would come down to airline preference.

what is the route?

I don't believe there is a J cabin on 737s

F is called J on an int'l flight.

And they may even be on the very same aircraft imminently

Pick the one with the best time and routing option.

US F is the same as AA J.
No brainer, AA any day of the week.

US Air first class? or AA's 737 business class
 

Well i am a AA frequent flyer and i want to try other airlines. I have the choice of flying direct back home on US Air 757 on Envoy (first class) or flying SJU-MIA-PHL in B-class in 737-800

Well, you have the choice of earning AA miles on a more circuitous route, or earning US miles on a more direct route.

Envoy is much better than AA business (domestic first) class.

Does US even serve a meal on SJU-PHL?

Just to be clear, the US Air flight is only labeled "Envoy" since it's coming from Puerto Rico. It isn't actually their Envoy (lie-flight) service. If you're deciding between US Air's domestic FC (which this is) and AA's domestic FC (which I'm assuming, since I'm not an AA expert), I'd go with AA. I haven't flown US Air in several years, but I was Chairman there for 5 or 6 before finally getting out. There is nothing to be desired about their domestic FC service. Unless something has changed on their SJU - PHL routing, that is what you're going to get.

No it's not, you're comparing international business seats with domestic first seats...

1. Yes, meals are served up front on PHL-SJU/SJU-PHL flights. As an example, there is/was a 2:45PM SJU departure that serves Lunch so it doesn't have to be that close to a traditional "mealtime" to get service. You can do a sample booking online to see for your specific flight.

2. US Airways schedules the International 757-200 and sometimes the A330 on this route. You get international (Envoy) seating (160 or 170 degrees on the 757, lie flat on the A330) but domestic F-level service on these flights. Having flown on AA "business" tickets to the Caribbean, I'd avoid the AA 737-800 as long as you could secure seats on a US flight with Envoy seating (again, only the 757/A330). In my limited experience, you likely won't have IFE on these flights if that means anything to you but if you're like me, it's all about the seat. Fares seem to be the same for all these flights (regardless of aircraft) so the only caveat is that you won't get any sympathy from US for putting you on an A319/320/321 last minute due to IRROPS/unscheduled aircraft substitutions.