Welcome to the world of comparing ASHPs @tomguest. I have not been able to compare like for like - stated elsewhere. 
Im sure they are both good products.  For my particular home/solution I got Nibe, LG, Samsung, Stiebel Eltron, Mitsubishi and Daikin quotes. Nibe and SE were several £’000 more expensive. I was told they were “better” ASHPs. But looking at the data, I couldn’t see any better COP or SCOP. Better could mean more reliable, but you’d still have to pay for an annual service. So I ended up going with Daikin based on the installer and they offer a 10 year fixed price service and parts/labour warranty. 
Elsewhere there is a list of questions to ask installers for you to gather information to come to a satisfactory conclusion. 

Tom,

FWIW I would say that Grant  customer service is good (btw it is my review of the Aerona 3 on this site).

We've had some performance issues which eventually proved to be related to the design and installation of our system, not the ashp itself.

Although this has all been 4 years since the system was installed, Grant recently made a complimentary service visit to help identify the problem.

Getting the design and installation right is essential. An MCS certification doesn't guarantee that the installer really knows what they are doing.

Also, make sure you have a surge suppresor on your household electrical supply.  Our rural location is prone to micro power cuts and voltage spikes which eventually blew the main PCB on the heat pump. An expensive repair. We now have a surge suppresor fitted at the fuse box which protects the whole house.

Mike

Thanks for the replies.

The company I think I will go with (Climate save renewables) are giving me a 10 year workmanship warranty, 7 year product warranty (standard on Grant) and 7 years free servicing.

I too am wondering this question - in this case Grant v Mitsubishi.  The installer says Grant 'is the better product' which could be installerspeak for 'they make us more money'. They will do Mitsubishi if I pushed for it (and if the supplier has stock - not sure what the situation is).

I note the sound pressure levels on Grant are 7dBA higher, which is quite a lot. (installer said 'it's quieter than a washing machine' - I really hope so!).  I need to do some more studying of the differences.

Generally I'm more comfortable with an appliance from one of the big A/C manufacturers - I've worked with A/C systems in ~50C daytime temps and only the big Japanese and Korean brands (plus Carrier) are up to it. I'm a bit wary of companies that really focus on gas boilers and suddenly develop a line of ASHPs, especially sales pitches like 'made for the UK climate' (without describing what that actually means in real terms).

Interestingly, I think I figured out that Grant are rebrands of Chofu of Japan - the diagrams in the installation manuals are identical.  I'm not familiar with Chofu but I do wonder about long term support and parts availability.

@prunus, a valid point. Will that not become Grant's problem to get spares or keep them in storage? We've been in contact with all the big heat pump manufacturers and I've been incredibly impressed with Grant's all-round professionalism. Ironically, LG and Mitsubishi have been the worst. In the greater context that probably doesn't mean much, but I can see Grant dong more to support their customers than other Asian manufacturers.

@tomguest, did you finally make a decision?

@prunus, have you considered a RED. Made in Ireland for a UK climate?

@editor I went for the Grant via https://climatesaverenewables.co.uk/ - The cylinder and electrics were installed a while back. The actual heat pump is being installed next week. So far I can't fault the companies service. Hope the heat pump lives up to expectations.

Interestingly we are getting lots of people on our street asking about both heatpumps and the 100mm external insulation we have added to the side of our semi-detached house.

I did look at RED - Freedom Heatpumps have some videos on them.

I'm a bit wary about this 'designed for the UK climate' thing: I've seen that slogan used as marketing before (on a dehumidifier) and I have to question what it actually means - what's materially different about the UK climate compared with other parts of the world, and how is the design adapted?  Need to separate the facts from marketing based on essentially xenophobia.

Anyway, RED looked OK from the videos. I liked the vertical design with the big fan although I didn't have a good feel for costs (none of the usual 'merchant' sites seem to list them). But my installer is one of those plumbing and heating companies that likes everything to be like a boiler, which is why they like Grant (and, to be fair, the Chofu/Grant heat pumps look quite nice in that respect - electricity in, hot water out, uncomplicated plumbing and wiring).

I pushed for Mitsubishi, based on the noise figures, and they were happy to quote for that. But the lead times on cylinders were out to 12 weeks which was a problem, so we ended up back at Grant.  My solution to the noise issue is to go for a 13kW Grant (double fan) rather than the 10kW Grant (single fan, noisier) and potentially use 'night mode' to reduce the fan speed if the noise is a problem.  The actual heat output figures aren't enormously different so I don't think it will be massively overpowered, although I'm a bit worried by minimum flow rates (our heat loss came to 5kW according to their calcs, 8kW to mine, but I think the extra power might come in handy given low thermal mass in some areas). Hope it doesn't affect COP too much.

Anyway, installation is booked for a month's time. Which is handy, because our oil boiler has unilaterally decided to become carbon neutral.  Which means it's rather cold at the moment...

@prunus,

good progress but shame about your boiler!

Our MCS heat loss calcs were about 6.5kW and we have a 14kW Mitsubishi. It has no problem modulating down to lower outputs and is very quiet.