Hi Dave, and welcome to the forum.
I've had two 3.0Sis and bought my current M just over a year ago.
The 3.0Si has a great engine that revs to 7,000 rpm, but the M has a masterpiece that revs to 8,000! All of mine have been Coupes, and based on those I'd say the 3.0Si is maybe 80% of the car for 50% of the money - but if you want that last 20% and budget is doable it's well worth it IMHO!
As for issues, the 3.0Si can suffer from oil leaks from the cam-cover gasket and oil filter housing, and has an electric water pump which will at some point die which leaves you stranded. How long they last is variable - my 1st one needed a new one after 60K miles but my 2nd seemed to be on the original when I sold it on 91K miles. A new OE pump by Pierburg costs around £250 (or £500 if you buy from BMW)!
But the M needs much more TLC with every other service being an Inspection 1 or 2 with oil serivces in between. Inspection services involve checking valve clearances and adjusting them with shims, if required, because they don't have hydraulic lifters. And at some point it is worth getting the crank rod bearing shells replaced because they do wear. How quickly seems to depend on how the car is driven, especially when the engine isn't fully warmed up, but now they are all over 10 years old it's hard to know. My car had 7 previous keepers so I got mine replaced on 77K miles and was glad I did when I saw the old ones - budget around £1,000 for that by an Independent, although I got new engine mounts fitted within that. Differential seals sometimes leak, and of course brakes, etc. cost around twice as much because of the "M Tax"! And Road Tax is £580 a year rather than £330.
Anyway which suits you better may depend on how you use it. Mine have all been "special occasion" cars, but if I was planning on using one every day I'd choose a 3.0Si mainly because an M might feel a bit wasted commuting - if anyone does that any more!
But the M just feels so much more special even when I start the engine before going anywhere! Plus the M comes without the run-flat tyres that the 3.0Si has, Hydraulic PAS instead of Electric PAS and an LSD. And it has a ZF gearbox which I have found better than the Getrag boxes in my 3.0Sis.
Also bear in mind these are pretty rare cars. Only about 600 M Coupes and 600 M Roadsters were registered in the UK, less than 3,000 3.0Si Coupes and around 1,000 3.0Si Roadsters (and some of the 3.0Sis were Autos) so you need to be prepared to travel.
Nearly forgot, but now they are getting on in years they can all suffer from corroded rear brake pipes. BMW routed them both under the floor on the passenger side in a plastic cover, but it ends a bit in front of the rear wheel and can end up with a build up of road debris that will retain moisture. New OE ones are pre-shaped so the fuel tank needs to be dropped to fit the offside one in the clips on the bulkhead, but if replaced in copper/kunifer it can be poked through then shaped afterwards.
Finally if you are buying a Coupe open the tailgate and check all around the edges as they can rust, and the BMW warranty will have expired by now.
Hope this is useful - I'll finish with a few photos for inspiration! My 2nd Coupe, my current one and my old bearing shells.
North West Surrey
