The 2 most frequent carriers out of SJC is AA and Southwest, but you must know this if you fly out of SJC a lot.

For frequency/availablity of flights from SJC, AA would be the clear choice if you want an airline that goes international. If you really want to fly mostly out of SJC, that's probably your only real option. United often flies/drives you from SJC to SFO on flights that originate in SJC, you have to be a little careful there. That said, if you don't mind SFO, you have a lot more flight/carrier options, including all the international airlines.

There are many people who love the AA AAdvantage program. I used to be one of them, but AA recently instigated a $100 change fee for FF tickets. It used to be you could change date/time for free(flexibility I really liked), but not anymore on AA. Other airlines may(NW) or may not(Delta/United) charge a change fee as well, and rules are always changing. AA FF awards are definitely available if you are flexible. I personally have used AA for FF tix domestic 48 US, Hawaii, Carribean & Central/South America, but not to Europe, Asia or Australia. For Europe I paid for tix or used NW FF program. I tried to use AA for Alaska, Europe and Australia in the past, but couldn't get the dates/times I needed so I just paid for tix. As for upgrades, AA doesn't upgrade well at all --- I've never been upgraded in my entire AA life. Others may have better success.