If your profile information is correct and you are in Chicago, that gives you three options:
1) United (ORD)
2) American (ORD)
3) Southwest (MDW)
I am (or rather will be after my flight today) Gold on United, so I can tell you about their program.
This biggest perk United has over any other airline is Economy Plus, although Delta is starting to add these seats now. Economoy Plus seats are regular economy seats width-wise, but have much more leg room (typically 36" which is 5-6" more than standard seats). As a United Premier member, you have access to these seats, but when you get access depends on your status level.
If you are Premier Silver (25,000 miles or 30 segments) you get access at check-in. I've yet to have a flight where I couldn't get into Economy Plus, but have had to settle for a middle seat a few times and you have to be on top of checking in right at the 24 hour mark. If you are Premier Gold or higher (50,000 miles or 60 segments) you get access at booking. Seats always look to be available then.
You can also look forward to automatic upgrades to Business or First, but don't expect it too often if you are only Silver. I've had over 50 flight this year and only gotten upgraded twice (and one-time my connecting flight was cancelled due to weather, so I didn't even get to enjoy first class).
Other United perks for Silver members are:
- Reserved check-in counters at most airports
- 1 free checked bag (bumps up to 3 with Gold)
- Dedicated Security lanes at most major airports
- Access to TSA Pre-Check
- Early boarding (group 3, group 2 for Gold)
- 25% bonus on miles earned (50% for Gold)
The special check-in and security lines have saved me before during really crowded travel times (like trying to get out of NY the day before Sandy hit). While the regular lines were an hour or more, the Premier lines were 15 minutes at most. Boarding early means you never have to worry about overhead bins being full.
The other benefit to United is that they are Star Alliance partners with US Air, and although I always hate flying US Air, it's nice to be earning qualifying miles while you are doing so.
The one downside to United is they are stingy with their club access. No level of status gets you unlimited club access. With Gold and higher, you get club access when travelling internationally. I'm not sure how other airlines work.
A note on earning miles, there are two kinds of miles with United (and I assume other carriers). There are the miles you earn that are redeemable for free tickets or upgrades. These get the 25% bonus and can be bought. Then there are Premier Qualifying Miles (or as I like to call them Ass In Seat miles). These are the miles that count towards earning status and only count when your ass is in the seat. You can't buy them. You can't even buy a ticket and then not show up. You only get credit if you actually take the flight. The 25% bonus doesn't apply to them. The only perk is that if the flight is on United or United Express and you have status, the minimum amount you can earn per flight is 500 miles.
I like United's program a lot. Are they the best airline -- no. But if you have status, they are pretty darn good. The fact that they are huge and fly everywhere doesn't hurt either.
One word of caution: with United having a major hub in Chicago, there will be a ton of status members flying through there (either departing directly or connecting through). This will make Economy Plus and upgrades pretty scarce at times.
I have a friend who flies Southwest a lot and had status with them, but I'm not sure what its benefits are. I think priority boarding and free wifi if you are high enough, but I'm not positive.