Sasha: United is not as generous as American is with award travel to Europe. You are (in principal) allowed one stop-over either on the outbound or return. A stopover is any break longer than 24 hours. If you plan on going to a destination not flown by a United plane, your miles will only take you to where United flies. You will have to cash in additional miles to fly to the final destination on Lufthansa. American will allow you to fly anywhere in Europe that British fly's without costing you more miles. Due to this, it is much easier to create a stopover with American miles than United miles. If you are looking at doing a stopover in the US between your departure city and destination city you will have no problem(as long as it is in the routing).

If you are very specific as to your travel dates, I would reserve now. If you are not picky and are going to a destination with lots of United flights (ie London) you should have no problems finding award seats up to departure time.