This question is funny; this is almost the perfect question for a shill to ask to highlight Solaris 10 new features but no one gave the pro-Solaris answer.
This is a textbook application of Solaris Zones. The shared kernel provided by Zones lowers the overhead of virtualization, and increases speed dramatically. If you have an idea of a standard install for VPS (bash, apache2, php5, python 2.X, ...) you can create a single "gold" zone to use as a template to clone to new zones. Package repositories are available at sunfreeware and blastwave providing you with pre-compiled packages, removing the need to compile your own if you don't want to.
You can create your template, charge $X per VPS and clone the template for each new customer, total config time upwards of 5min, 0min if you script/automate it. Upgrading the "global" zone (the base system) will cascade those upgrades into the zones, or you can upgrade per zone, also highly automatable.
Solaris has kernel space accelerated SSL encryption for supported hardware: expensive cards, Sun/Oracle Niagra2 CPU based systems, and the new Nahalem systems with AES acceleration, which greatly increases the number of SSL protected websites you can host per system. http://www.c0t0d0s0.org/archives/6926-Performance-Impact-of-kssl.html
Solaris 10 has many new features in resource management allowing you to segregate individual zones/processes/groups/users and keep runaway or compromised applications in one zone/group/user from impacting any others, as well as all the normal POSIX resource controls on memory use, file descriptors, etc.
Solaris 10 Zones (and Solaris 10 in general) was designed from the ground up to prove excellent security, accountability, resource management, and to dovetail nicely with Sun (and now Oracle) hardware offerings. When released the Sun T5240 + Sun Solaris + Solaris Zones package was the best platform for page views per second for the money.
In terms of technical merits, Solaris Zones is probably the best VPS solution available. But as is usually the case the issue is requirements and costs. Licensing, support costs, and Niagra2 or newer CPU hardware costs are rising with the Oracle takeover.
So evaluate the following: Will the higher VPS density, better VPS isolation and wiz-bang features compensate for higher licensing costs (if using Oracle Solaris), smaller user base to draw peer support from, higher hardware costs (for SSL accel), cost of supporting yet-another-OS, cost of hiring people to support yet-another-OS, the longer time it takes for security patches to get released.
If you already have a windows team, do you really want to hire a Solaris team just to shave a few percent off of your hardware bill? Stick with Hyper-V until it'll save you money to switch. If you already have a large deployment of Solaris systems then go with Solaris. If you have a large Linux skill pool to draw on, do a Solaris trial and see how much extra time it takes 3 admins to learn the differences and maintain a new environment for 6 months
But technology should almost never dictate your business decision process. Much as I hate to say it for most service providers it makes more sense to provide a Windows based VPS system than a Solaris one. Unless you know now that you're going to need the feature set, and the advantages are going to save you lots of Time And Money(TM) you probably don't want Solaris.
But if this isn't for a business and more about having fun, then go ahead, use Solaris! It's alot of fun, has tons of features and options that you've never even thought of if you're coming from a non-commercial Unix background. The deeper you get in to Solaris the more you learn about smart engineering and new ways of solving technical problems. I've yet to see a Linux box with a "load average: 1000.0+, 1000.0+, 1000.0+" that was responsive and easy to recover.
@symcbean: I know Solaris (or Slow-laris as it is sometimes called) has a reputation for poor performance (eg your fork example) but I seem to recall that the "Solaris Internals" book said that they re-engineered the threads significantly for Solaris 10, and process creation/forking performance was among the industry leaders. The LWP framework where each thread in an app is mapped to it's own light weight process in kernel space apparently gave a big boost to performance, reliability, and accounting. The big hurdles for Solaris aren't so much technical as operational (bad ui), cultural (small user base), and political (Oracle).