Don’t get me wrong I am a huge BMW 6 cyl fan but I think the market, especially for used, has moved on and just doesn’t like bigger engines once out of warranty – look at the M3 V8, they are going for peanuts and I reckon in 2 years you can get an E92 M3 for the same or less that an E46 M3. BMW are doing huge deposit contributions on the M3 V8 that tells you the (new and used) market doesn’t want them anymore.
I think the M135 will always be more “niche” going forward and time will tell if that leads to strong residuals or not – there are quite a few on the BMW approved website and knocking on £25k already but I certainly don’t think you can compare it to the Focus RS which was much more limited production number.
I will certainly look at an M135 if the lead time on my GTi gets out of control and I also totally agree (and have always said ) that at £25-6k new the GTi works but at £30k + its too pricey in my view.
I do think that the wider appeal and perceived lower costs of running a Golf help it longer term in terms of 5 year + residuals, especially compared to bigger engine cars, provided you don’t go silly on the options list – which goes for both cars ! You can spec an M135 up to £38k easily and a golf up to £33k if you want. It also depends how long you keep a car ! if you get rid after 2-3 years you will always be stung, if you keep it 5-7 years it will level off..
The day to day running costs of the Golf will definitely be lower though – road tax, insurance, slightly better on fuel and these are the things people look for on the used market. When I bought my 330ci 3 years ago the market had just turned and they couldn’t sell 3 litre petrol cars on used forecourts, I got it for the price of a similar age Golf Tdi which cost £10k less when new.