interesting idea, never really looked at sim only deals, I had just always upgraded every 12 months when my contract ended. The only reason I stayed with t-mobile was because I was on an older contract and my data was truly unlimited
goes off to have a look around...
It does seem like a good option if you want to upgrade more often. 24 months is becoming way too common now, 18months are a bit pricey too to begin with.