I had to pick between the two a year ago and picked YieldSense.  Had both a 20/20 and Integra already.

20/20 setup a bit more expensive, as I recall.  They argue it is more accurate than any other monitor on the market including AgLeader, I can't tell you that.  I am very pleased with the ease of workflow with the 20/20, especially calibration/validation and post-calibration.  Pretty slick automated comparison of planter hybrids and yields.

I don't know that you are required to upload your data to Fieldview to use a 20/20 - just think that you can choose, if you wish to.

I am sure AgLeader is easy to use, I've just never been as familiar with it as with our 20/20 and Trimble gear.  I am sure you can achieve similar workflow if you plant with AgLeader, or use SMS or some other GIS software and overlay the maps.

What GPS signal will you use with your combine?  I set up an old EZ-steer and pick corn with RTK w/o row feelers and surprisingly pretty good!