I am biased but I think the US approach is much better. You start the MS and PhD at the same time. You get real research experience during the first two years,

I am biased but I think the US approach is much better. You start the MS and PhD at the same time. You get real research experience during the first two years, while also doing coursework: if things don't work out, you have an off-ramp in getting a terminal MS (usually for no financial cost.) If you keep going, then ideally you have a good sense of what research you want to do. You may have one or more publications/near-publications under your belt. Now you write your proposal and finish out the dissertation over the course of 2-3 years, which is realistic. I may be missing something about the UK approach but this seems much more reasonable to me.