I think I'd go for the clear plastic Amanda - if you set your plants out and the soil is damp ( not sodden ), the plastic will keep the soil moist enough at this time of year - usually !! You will need to check tho - if you have exceptionally dry periods, you may need to water.
It depends on what the ground is like ( free draining, moisture retentive ? ), the size of the plants you put in the ground ( titchy or well rooted ? ) and exactly which plants you are doing.
If you use plastic, just make sure that if you have some warm days ( you never know your luck ), that you "lift" the ends of your tunnel to ensure ventillation.
Sorry if it all sounds a bit vague but if you are beginning in a new location, it is often a case of suck it and see.
I'm in the SW UK and whilst I can still have the hardier salads outside with no covering at this time of year, I also do troughs of salad in my cold GH which keep me going, more or less, until I get the spring stuff outside.