I tossed this around for a while.
Flatplate:
Has the ability to transfer more heat more quickly
May need a circulator if you want to heat an entire water tank instead of heating just on demand
Can clog quickly from hard water and mineral deposits (plumb it up so you can flush and clean it yearly)
Sidearm:
Can circulate via convection to heat an entire water tank in about 15 minutes
Usually not quick enough for on demand heating
Rarely clogs from hard water or mineral deposits
You can build your own or buy several different types of pre-made ones
From that, I chose to purchase a sidearm with a rolled interior tube from freeheat4u on ebay. I figure if that doesn't work well enough, I'll build a second one myself and run them in parallel. I have hard water, which was the main reason I went with it. Once I'm done installing everything, I might look into a flatplate for on-demand heating later on.