Hi tigger,regarding pricing
I disagree slightly with Jill and Randy on the price issue. I think that for most commercial sites, the CMS itself is not going to be an important determinant of cost. Propriety CMSs, if they are intended to be deployed in cheap sites are priced accordingly. If you are spending £45k, I assume that this means the site will require a lot of design work or a lot of programming/customisation. Simply skinning a CMS, open source or proprietary is not that expensive. There is always the possibility you are being over charged, but I'm assuming you are not since you seem to have multiple quotes.
I work mostly with custom CMSs, but would not assume you are getting a better deal from developers using a proprietary CMS. They are not developing it for you, nor is it usually an advantage if they are. You are mostly for time, software is usually not expensive. Regarding Joomla
I don't have many hard opinions about Joomla. It's the right tool for some job, such as news sites. Plenty of capable professionals that have my respect use it out of preference. Plenty of large sites run on it. I certainly don't think it should be disregarded out of hand. It's not the easiest or most intuitive back-end, but it's OK.Regarding Wordpress
Wordpress is obviously a great blogging platform (to my knowledge, better then any proprietary one). It really is great software, but I do not think it is right for non-blog sites unless they are very simple. I think the the reason people do use it for this is because they know Wordpress and don't want to learn a new platform. I don't like discounting it completely because I am sure there are times when it is exactly what you need, but.. it's just made for a certain types of site. WP is not a general purpose cms.Regarding Future proofing
Both platforms are probably here to stay. There will be future updates. I have heard that joomla has a history of problems upgrading due to plugin incompatibility. You don't have to upgrade though. My personal preference for future proofing is hosted platforms. Upgrades are monthly (sometimes daily) & painless. So you will have access to new features as they become available. Hosted solutions have downsides too, so you need to have a good idea of your needs. If you need a lot of customisation, it may not be for you. That's where consulting from your supplier comes in. An experienced consultant will have a good chance of guessing what your needs will be in a year or three, which is a big part of future proofing.
On a small-medium project, your future proofing is more dependant on your support. If you switch developers/consultants, it will often be better to start from scratch (or near it) regardless of the platform you use. You will be able to find other companies that will work on Joomla or WP, but changing hands can be tricky. Regarding SEO and other issues
Both platforms are pretty good out of the box and can
both produce good outcomes. Being open source and supporting plugins (extra bits you can install that add or change something), they have an answer for almost everything. You will probably find a plugin for every SEO religion
Plugins are all over the place. Some are proprietary. Some are open source. Some are great Some are terrible. Anyone can write a plugin, so naturally the variance is huge. A google maps 'mashup' is likely to be mostly something that is developed as necessary. Google maps API is essentially the plugin. Any plugins using it will usually be just providing you information (eg coordinates) to feed it. But obviously this depends on what you mean by 'mashup.' Making the Right Choice
It's great that your doing your research and getting opinions. But understand that all the CMSs that are popular are popular because developers actively choose to use them over other choices. They all have certain appeals or advantages. You will always be able to find fans of each one. The advantages are usually either highly subjective (eg: it has a better plugin architecture or a cleaner codebase) or highly specific (eg: supports Open ID for comments).
Again, I think you need someone knowledgeable that you trust to advise you. Judging by the price-point, I assume that you are getting into a substantial project. You need to be able to get good specific advice on it. Don't worry about the tools they use too much. Make sure that they help you develop the idea of what your site needs to do.
BTW, if you would like a counter quote or specific information, send me a message via the forum messaging.
Hope this helps,