My opinion, but interested to see what others think:
Tunnel temperature will fall at night faster than glass - so if heating it then glass may be better. If heating it "often" (many nights) then insulation will be important anyway - typically bubblewrap. Dunno if that is easy to do in tunnels, plenty of framework in greenhouses to attach it to.
If risk of damage to/by kids then greenhouse can be clad with safety glass, or polycarbonate (although the later has a habit of blowing out in high winds, typical solution is to silicon the panels in, but that makes them hard to remove if that was ever necessary)
Ventilation easier and more controllable in a greenhouse - however I think that most of the models I see have too few vents as standard. I think a 10' long greenhouse needs 4 vents, and preferably a louvre (in the side, or end) as well so that convection cooling can be used most of the time (in heat wave it will still be necessary to throw the door open for forced draught cooling, which increases transpiration stress for the plants. Easy to attach automatic openers to greenhouse vents, if you are away for the weekend, or working shifts, not sure if that can be done with tunnels?
Greenhouse vents can be set "ajar" to allow gentle ventilation - useful to keep a small airflow to prevent moulds etc. in cold weather. Similar can be achieved with tunnel where sides can be raised (usually insect netting is installed on the sides as well) but I think that introduces cold air close to the plants. Might be more of an issue in a tunnel if Paraffin / Bottle Gas heater is going to be used as they generate a lot of condensation.
Cropping near the sides of tunnels can be difficult - worth choosing a model with "near vertical" sides
Tunnel has ongoing, but infrequent, labour effort and cost of replacing the plastic.
Alternatively early seed raising might be achieved using artificial lighting in the house or garage. No issues with temperature (or a dreadful late spring with dramatically reduced light, like Spring 2013), but as soon as the plants get to a certain size they need to move to something more spacious. Supplemental light, indoors, won't help with protected crops in the Summer either, but might be worth considering instead of heating the greenhouse / as much.