Touring - another sizing question

I am looking to purchase the schwinn moab 3. I'll be using the bike almost exclusively for touring. So, my question- I spent a few minutes on the 19" frame, which felt fairly comfortable and had a standover height of about 4". I was told I would possibly be more comfortale on longer rides on the 21" frame, which I haven't found anywhere to test but can be ordered. Any advice on which of the two to go with? As reference I am 6' tall with a standover height of between 33 and 34". Any advice would be greatly appreciated.

The standover height has little relavence to comfort, it the length of the cockpit which matters.
Do you ride a bike at the moment, is your position comfortable for touring ? If so, you need to replicate it on any new machine. Measure the distance from the nose of the saddle to where you hold the bars.

Some tourists like a long stretched out position, others prefer a more upright one, it depends on your age, flexibility and riding background.

If you dont know what size you like, then try and borrow or hire a bike for the weekend, and see if you like it. Even if you dont, you should have some idea why.

NB bike "size" is measured from the bottom bracket to some point at the top of the seat tube. The exact top point varies and the bottom point may be a different height off the ground between 2 manufacturers. For the same "sized" frame, the length of the frame may vary with the style and intended purpose of the bike. What does this all mean? Take a tape-measure to the shop and get some meaningful dimensions for yourself.

As far as standover height goes, you are looking at approx 1.5 -2.0 inches of clearance between you and the top tube...you being 6', should probably go for the 21" frame,and then sort out things like stem length and rise etc , etc,