A couple of disadvantages - they likely won't respond very well to load spikes, and your water treatment had better be nothing less than "excellent" at all times. Ask me how I know this...
I didn't say it was good, I just said it was cheap. If the goal is to have a good reliable unit, pick the first one. If the goal is to save money (initially) pick the latter.
Me, I think in the end you usually end up getting what you pay for. But some times good engineering doesn't win out, good bean counting does.
