Which to get.

MICHELIN PILOT SPORT 5 275/45R20 110Y

or

MICHELIN PILOT SPORT 4 S 275/40R20 106Y

Will the Pilot Sport 5's fit as I am not able to get them in 40 profile only 45. Any wheel arch rubbing.

The 45 profile will fit with no issues.

Cheers
Col

Remember, both models are ultra-high performance super-sticky tyres usually fitted to M-BMWs, Porsches, R-Jaguars and any other low-slung, high-speed vehicles. While the grip and performance are splendid, a 2.7 tonne RRS will eat through them in no time.

A 45 profile is actually preferred due to a slightly taller sidewall which provides additional comfort without any downsides. Regards etc.,

Wojtek

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WAS: 2006 RRS Supercharged
IS: 2010 RRS TDV8 HSE

What do you mean eat in no time? On my last two sets of tyres I got 35,000 km (22,000 miles)per set. My average speed is around 26km/h and I do about 5-6000km/year. So as long as I will not get much lower km per set it will be OK. A bigger problem is tyre deterioration (ie cracking in the sidewall) due to heat and UV light.
Also my current set of Goodyears became extremely noisy after a couple of years of use.

With such mileage/speeds I'd ask "why an UHP tyre then?", but I tend to overprovide for my cars as well.
I only have experience with an M BMW on 4s and they were gone within 15000 kilometers. Mind you, they are overkill for a non-M BMW, but you can squeeze a lot out of them because of the grip level and temperature response when on track - they are trackday car favourites for a reason.
For a very rough life expectancy estimate just compare the treadwear ratings of your previous and future tyre. 4S has a treadwear rating of 300, if I remember correctly. I had the same 300 treadwear rating on my Pirelli P Zero SUV tyres on my previous RRS (2006 SC) and they were done on the 24000 kilometers mark.
The Michelins get a great tread warranty, though, so maybe worth a shot, even if you will rather not use them in an environment and manner they were designed for. But heck, it IS a great tyre, even when cold and not on the track  Regards etc.,

Wojtek

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WAS: 2006 RRS Supercharged
IS: 2010 RRS TDV8 HSE

I would be happy with 24,000km. Thats 4.5 years life with the km's I do.

My only concern is the noise degridation after a few years use. My last set were Michelin Latitude 3's (not Goodyear as previously stated) and they became very noisy after a couple of years use.

Most tyres get noisier at the end of their life, just turn the radio up a couple of notches.

Cheers
Col

If I go for a 275/45 R 20 tyre what size tyre to put on the spare which currently is a 19" rim with a 255/50 R19 tyre and will it fit in the spare tyre position under the rear of the vehicle.

You would probably need to go up to a 255/55 x 19" tyre if you want to keep your original spare rim, it is actualy possible to fit a 275/45 x 21" wheel with a brand new tyre fitted in the spare wheel well if needed. The 275/45 x 21" is about 781mm diam. 255/55 x 19" is a round 763mm so plenty of room, the 275/45 x 20" is a little less at 756mm.

Cheers
Col