You are not the first to experience this frustration! The small business telephony market is a complicated one, not only because there are so many vendors in the market, but also because (as you mentioned) there are a number of platform options as well. To add to the confusion, telephony vendors are hardly quick to provide a true landscape of the market and how their product (and platform!) compare to all your other options.
As far as which option makes more sense for your business, it's hard for me to give you a definitive answer without more details. But here are a few things to consider:
You're a law office moving to a new location. Depending on how new the building is, this may afford you more flexibility wiring-wise. (You may be able to easily install a T1, piggyback onto/ partner with another business' cabling, for example.) Location also pertains to geography. Although internet connectivity is growing and evolving, not all locations are equal. For instance, if you're in an extremely rural area, it may be harder to configure the proper wiring to handle a VoIP platform with the reliability you're looking for.
Assuming Location is a non-issue for you, and you decide to go VoIP, there are platforms out there that are truly quite reliable. However, to ensure 99.99% reliability, you often have to pay extra for the redundancy. In other words, if you go with a hosted VoIP solution for example, some vendors require buyers to install a T1 line regardless of your current infrastructure. This adds to costs. Your TCO is a huge consideration in general when evaluating the market because different vendors have different pricing models. (I'll get back to this in a minute...)
Hosted vs. On-Site VoIP:
This decision must be weighed against a number of variables including your long-term business plans (growth), learning curve, resources available to run the system internally, budget, integration/ interoperability requirements, feature requirements, customization requirements, and the importance of vendor locality (proximity)... among other things. Without going through each of those, the main (and most impactful) differences lie in *costs, *support, *scalability.
1. Costs: with a hosted solution, one typically pays a relatively low up front cost, but continues to pay over time forever (effectively leasing the service). When purchasing a system to own on-site, the up front cost is much higher, but then the system belongs to the business. Other cost considerations lie in infrastructure (can you leverage your existing hardware? existing wiring? etc.) Some businesses offer a full proprietary system (software, hardware, etc); Others offer software only, and the customer is responsible for providing the hardware; Others offer the software and integrate with a number of specific hardware models... Each model has different implications for the expenditure required. Other cost considerations with on-site solutions: upgrades, licensing, feature add-ons, scalability, etc. With hosted: consider costs surrounding monthly fees, feature add-ons, and providing the proper ISP connectivity.
2.) Support: with a hosted solution, the vendor runs and monitors the system at all times. They are also (obviously) to go-to when the system goes down or if the user has any issues. General learning curve is a consideration regardless of which system you choose. With solutions you host yourselves internally, it's typically a good idea to have the personnel in place to manage and monitor the system. (This is another cost consideration). Support is also worth mentioning in the context of your company/ personnel's general experience with running a phone system and/or the propensity for unseen issues to arise. For many SMB buyers, locality of the vendor is critical. Having a local contact 'right around the corner' affords many businesses the peace of mind they look for in a phone system, VoIP or not.
3.) Scalability: With a hosted solution, service is usually easily scalable, but only to a certain point. Although some hosted vendors tout clients in the large enterprise sector, there is often a ceiling in actual application. An example of an enterprise application using a hosted service might be a call center at a single location, or leveraging the ease of hosted by integrating it with another solution. In any case, with an on-site solution, it's easier (and sometimes cheaper) to grow the system long-term. Many on-site buyers view vendors as a business partner in this context, looking to grow the business with the system and the system with the business. Expenditure-wise, this is often the more sensible approach if you're certain your business will be growing substantially in the short/medium-term.
In (a little more than a nutshell), these are the main considerations to make when considering hosted vs. on-site vs. analogue. There is another question here http://www.focus.com/questions/information-technology/what-are-top-pros-and-cons-hosted-voip-telephony/#38587
you might find helpful. In summary, it's hard to rely on vendors for a transparent and fair comparison, but these are a few considerations you can make with the knowledge you already have of your own business (requirements, roadmap, infrastructure, budget, etc.).
Best of luck!