If you can get the 46/36 Ultegra, just buy a 34t ring for it. Not expensive and it will work well. Personally, I like a 46/34. I'm using a CX70 46t ring on a 10 speed Ultegra crank. Would have been less expensive to go the other way but it's what I had. As others have mentioned, the 46/36 is the traditional CX gearing. They don't need the extremes of 50 or 34 and the smaller 10t drop is more forgiving in muck, less likely to drop a chain. In theory, anyway..