Ditto. I have both English and Metric sets of mandrels, plus a variety of knitting needles & steel dowels, etc. I tend to use metric more often than English (at the moment).
If you can't decide which to choose, pick a handful of weaves you would like to try and find the suggested sizes for those weaves. If the majority of the weave sizes are in English, buy the English set...if the majority of your sizes are in metric...then buy the metric set. Or, whichever system you are more comfortable using. *grin*