[GraciesGran]

A!lotment shop has snowball, Shakespeare and a red onion sets at £2.55 a kilo.

Which would be your preference?  I like the look of the snowball but have never grown onions.

[mumofstig]

I had Snowball sets last year, and after ordering them to Autumn plant, I read other sites selling them for Spring planting 
Rather than chance them, I kept them dry and planted them in modules in the greenhouse in January. I finally planted them out 17th March. They were growing well, some lovely and big, when the got mildewed so I lifted them early and dried them in a greenhouse. So not a full growing season but they would have got bigger but for the mildew....

[CHRISDONOHUE]

I would go for the red onions which normally are at a premium price - at Wilko they are roughly twice the price - which anecdotally are said to keep better.   You could easily mix and match different varieties to test which perform best on your soil and your growing regime.

[GraciesGran]

I quite like the idea of some of each.  That works out to a 15ft row of each, enough to give a decent comparison

[New shoot]

I have always found with autumn planted onions, that the red ones are way more likely to bolt and go to seed the following spring than the brown ones.  I have not tried the white ones.

You can harvest them as giant spring onions (or some call them green onions) before they bulk up.  Those are expensive to buy and for £2.55 a kilo, it is worth a go.  You could easily keep yourself supplied for weeks.  Just pull some early and leave the rest to grow on.  Keep an eye for any signs of flower stalks forming and eat those first.

If you have allium leaf miner on your allotment site, you will need to grow all of them under insect mesh, as they attack over wintering crops.  If you have part of the crop ear marked for early use you can cram them in a bit