Unless you have very significant demands, a larger unit should not be necessary. I have a 3200 sqft house and the TK-Jr can easily heat many zones simultaneously while still providing hot water. I do this by setting the thermostat on the unit at 167 degrees. Each of my 10 zone pumps pushes about 0.5 - 0.8 GPM. The TK-Jr can handle up to 5 GPM. The low flow is sufficient at the high temperature and is actually more effective. Hotter PEX gives up its heat more easily and it sits in the pipes longer this way. A high-flow pump at low temperature would take a long time to deliver heat to the floors. And on the DHW side, I have a mixing valve to bring the temp down to 115 to prevent scalding. The additional cold water mixing in lowers the flow required through the hot water heater for each fixture. So a shower calling for 2 GPM (at 115 degrees) might only need 1 GPM through the TK-Jr (at 167 degrees).
I would highly suggest going with the TK-Jr and see if it meets your needs. You can T off your plumbing for a future 2nd TK-Jr side-by-side if you really need more flow. Since the TK-Jr costs about $500 vs $1500+ for the H2, this would be the more economical method. Also, if you do need the additional unit (which I doubt you will), then it'll be a nice redundancy to have two in case one fails.
I used 12VDC automotive relays to trigger my pumps via the millivolt programmable thermostat signals. For each zone, I added an LED to indicate power on / heat not called for, another LED to indicate heat being called for, and then a toggle switch with another LED to indicate the pump is actually running and as an emergency cut-off. I put these in a wooden box with the LEDs and switches coming through the front face and added a couple of fuse blocks so that each thermostat is protected by a 1A automotive fuse (I'm powering the thermostats off my house batteries so I don't have to change out AAAs) and each pump by a 3A fuse.
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