20" bikes ( mods) are smaller therefore easier to throw around more , but 26" bikes (stocks) are bigger so they arent as easy to throw around as much but they have bigger wheels which means they are more forgiving than 20" if you get something wrong. so at 5ft8 a mod might be a little small, but easier to throw around-which is good for a beginner, on the other hand a stock would suit you well size-wise but may be a little harder to get started on because they are bigger, therefore harder to throw about, i hope this helps Luke