Uh-oh, I hate to buck Slim, but here I go!! I will start with a disclaimer. I do not, nor have not run a sidearm. I run a 20-plate plumbed in before my water heater. My breaker has been off since October 13. I have a family of 4, 2 adults, two small children. My wife takes 45 minute showers, I take 5. Kids get 3 baths a week-ish.
I like the plate over the sidearm if you have normal to above normal usage. Sidearms work for low use situations. There have been many guys on here that have had to add small circulators to their sidearms to achieve the volume of hot what they really need. This isn't necessary with a plate exchanger. The more you use, the hotter it gets! Sidearms need downtime to regenerate. I've been gone for 4 days before, and yes, when I get home the water is luke-warm. But, I turn on the tub, let it run for less than 5 minutes, and I'm back to full power!
My final illustration is this: I filled a 400 gallon hot tub with 110 degree water. I hooked up a hose to my hot side of my washing machine supply and let her rip for 2 hours. Bada-bing. You can't do that with a sidearm.