I was in a similar position. Moved from YYZ to IAH earlier this year. I decided to go for SE and 1K this year. Wanted the SE next year to empty my account out and soften the blow, but will focus 2014 on MP.

I do have E50 on AC for life so no issue with lounge access. I also still have a home in Toronto so don't need to meet the minimum spend levels for 1K that can be problematic.

As was already said, other than flights to Canada out of IAH don't expect many free upgrades. I've been a 1K for a few weeks now and have been 0 for 4. Everyone here has status and cheap flights are few and far between.

Beyond the free upgrades the best thing about United gold and above is E+ and the free same day changes and standby. I have saved a bundle by booking the cheapest flight and then changing at checkin to the one I wanted or created a last minute mileage run with a fun routing. They also provide great compensation when things go bad.

The challenge is that you will be starting with 0 on MP and it is painful during that period and is harder to earn miles without the 500 minimum. Your AC gold helps a bit but isn't going to get you a comfortable seat. For using miles, the lack of Fuel surcharges in MP is nice, but the new partner reward chart sucks and it is almost impossible to find saver rewards on United out of Houston. So may be a push between the programs.

If you are going to do a lot of travel in the US it is worth going for MP, but suggest you take a quick sprint with a big MR to silver (LAX-KWI-BAH is a good one) so you start to get your 500 miles and access to E+ at checkin. The rest after that is gravy.

If you're just going to travel back to Canada or won't meet the minimum spends then you may wish to stick with Aeroplan.

If you'd like any other tips on the transition let me know.