Yep, you definitely need to compare both sizes of trailers before you decide what you want. Dirk and I looked at both the 19 and 21 (as well as the 5.0TA) before settling on the 21.
We both felt the 19 was too small for us. We also didn't like how the bed was closed in on all of 3 sides and most of the in/out side. I like to hang my feet over the edge of bed, which I wasn't able to do in the 19. I believe the new trailers have a bit of a gap at the end of the mattress now, but it still wouldn't be enough.
Most of the time I'm using the trailer by myself and the bed in there works great for me. We ended up not getting their mattress, which we didn't like, and got a full size Select Comfort/Sleep by Number bed. Since it's a bit shorter than the frame, I added some foam cushion at the top (foam that used to be in the dinette cushions, but got replaced as they weren't firm enough) underneath the pillow area. The remaining area is filled with the pump for the bed, a sound machine on top of that (so no worries about hearing trucks and such driving by; although the Tundra is always between the trailer and road, so it's not like the trailer is right on the road), plus a small bin that holds things like cough drops, alarm clock, etc.
For getting out of bed, it's great to get some extra umph by grabbing on the end of the bathroom wall (door is open during the night).
I like having the bed right there when I walk in as I immediately sit down to take off my shoes (usually slip ons) and put on my slippers (can't go barefoot due to needing orthotics) and the shoes go inside the nifty shoe bin built by Charlie of RV Widgets. It goes in the underbed storage area that we had ETI add a door in next to the bathroom for.
My bed making style is pulling up the sheets and blankets. I do tuck in the excess bits along the end and bathroom side but that's it. Takes about a minute to do. Then I can use the bed to fling stuff on while cooking breakfast.
But, yeah, each size trailer will have their own pluses and minuses. Test 'em out as much as you can and then make your decision. Good luck!
Oh, I also use queen size sheets so it's lots easier putting that fitted sheet on. I do the same at home - king on the queen bed.