I make the flight on semi regular basis from IAH. Best route depends on your goals…most points, minimize flight time / connections, lowest cost, most comfortable flight, arrival time (i.e not in the middle of the night), etc.
IAH - DXB is the most direct route and the A380 is a great plane with very good service, but need to make the drive from DXB to Abu Dhabi which is about an hour 15 min depending on traffic...but its an interesting drive and a taxi will take you to your Abu Dhabi hotel no problem for about $100. Also staying the night in Dubai can be a lot of fun if you have the time.
If want to fly United to get the points then will need to make a connection through NYC, LHR or perhaps FRA and fly Etihad the 2nd leg and Etihad is a very good airline. But United will be the most uncomfortable flight on the worst plane with the worst service compared to any of the middle eastern airlines…unless perhaps you fly Lufthansa which is equally as bad…and as a general rule its good to avoid German airports & airlines.
Qatar airlines through Doha is also a great route and plane with great service, but again it depends on layover time and cost. Could also take Turkish Airlines through IST, and spend a night in Istanbul going either way if you can as it is a fantastic city. Recently Turkish has been flying an older 777 plane on the IAH-IST route with a very poor configuration in Business Class, but the economy seats and service look pretty good.
You just need to look and see what works for you.
