Millsy - You weren't at the boat show were you? If you were selection wasn't great but there were a few nice boats to check out.
I would personally pick Maxum over Mariah any day of the week.. I work in the marine industry (Wakeboard Towers) and having drilled holes in both and spent time with boats brands Maxum is far superior in my opnion - not just in style and solidness but in functionality too. Everythings better thought out in a maxum for servicing, working on, getting around and seating etc. I've owned many types of boats from Nautiques to Dories, to Maxums and I can honestly say that Maxum offers the best balance for what I do.
The Maxum brand is one of the most popular in the UK now; and for good reason - they suit out conditions the best. Sure if you want to spend more you can buy a Sea Ray, if you want to spend less and compromise on quality go to a Bayliner... In my opinion the Maxum offers a good all round balance.
Now, tips for buying:
If you want to buy a Mariah avoid a comapny called 'Boat Quays' like the bubonic plague - I won't say more online but can PM you details if required. The best people to buy a mariah from are a company called Stirlings marine up in Scotland - it'll be a drive for you but you'll save money and get a second to none service. Alan is the guy to speak to there - if you let him know Mike Dovey from Monster sent you he'll ensure you get the right deal
If you want a Maxum the people to speak to are Picton Marine - They are in Bridgend, S. Wales. Huw Picton is the man to speak to and I can guarantee that not one word of bull will leave his lips - he is the straightest bloke I know. He'll give honest, knowledgeable advice and if he hasn't got what you need he should be able to at least give you a few pointers on where to find it.
Re: you engine query - don't be tempted by a 3.0 litre; I know the pricing looks good but it's not going to give you enough horsepower. With 4 on board, some gear and a skier you'll begin to struggle. Ideally you want to fine a 4.3 MPI engined boat.
Hope that helps!