DS has been in the (large) cot for ages and is perfectly happy there (just 3 )
But we would like him to progress so he can get out to the toilet on his own when that time comes.
We have room for a full-size single but are planning to move house next year so don't know.
Also have another DC on the way so will need a bit more space, but all the junior beds we have found go up to age 5 which means we will have a cot, junior bed and then will need to buy a bigger one in 2 years anyway.
I've done both (simply because for dd1 someone gave us a single bed and we were too tight to pass it up & someone offered to pay for one for dd2 and we saw a lovely extendable 'toddler bed' in Ikea) and I can honestly say it made no difference to how well they settled, but mine were about 2 when we changed them to a bed so I don't know if that makes any difference?
Guessing it depends how old they are too. My DD is 11mths and I dont think it will be long before shes trying to escape from it! It which case we may go for a toddler bed as a single bed would just swamp her.
As DS is 3 id say Single bed to save you the money in a few years time anyway.