Like you say, similar power and similar power delivery. Both a lot of fun, BMW rear drive is interesting on a damp road if you're not used to it. Both have huge followings and spares are readily available. The 318iS is much sought after (as are all Golfs of course!) so beware anything that looks too good to be true.
If forced to choose one over the other I'd go for the Golf as the interiors are much harder wearing - but I'd love to have a nice example of each on my drive!