Here's a great chart with the different comparisons on the machines:http://www.paperthreads.com/bonus/paperthreads_machine_compare_112008.html
This is a kind of hard question to answer because there is a lot of information on these cutters. It really helps to be specific on what you need.
Nowadays you need to treat die cutters as if you were buying a sewing machine. The information, differences and price points on these cutters are that vast.
If you type in my screen name you can find several posts on my comparisons and what these machines do. I also have some very long detailed posts over at SCS. Same screen name.
Also the Cricut MB is a wealth of info too. There are a couple of Craftster girls that post over there including me.
This is going to be really long. Sorry everyone. Drink time.
The three Cricuts are: Personal, Create & Expressions. There is also a fourth called the Imagine. Imagine is the Print & Cut machine from ProvoCraft. You have to use the cartridges with the Imagine. There is no third party software support and probably will never be at this time. I almost forgot we have the Cricut Cake and Mini Cake machines too. I will give you specs on the three most popular.
Personal- Known lovingly as the baby Cricut was the first version. No frills basic bare bones Cricut that cuts at 6x12. This machine is being phased out.
Create- Has half the features of the Expressions. Also cuts at 6x12.
Expressions- Has several features such as fill the page, center point, landscape, portrait, multi-cut etc.. Cuts to lengths of 12x24. Most popular cutting size is 12x12.
You asked about the Silhouette. I do have one. Graphtec machines were my first die cutters starting with the Wishblade.
Silhouette- Is a lightweight cutter at under 300 grams per pressure. You will only be able to cut lighter materials as seen in the above chart. It does have the three dot registration which makes for accurate Print & Cut.
It is more accurate than the Print & Cuts done on the Cricut's with third party software. This is because of the laser registration. The Cricut Print & Cuts uses a hinge method so there is always a bit of an offset.
Also the Silhouette does work on Mac, Linux and Windows. Silhouette can cut from all the vector drawing programs such as Inkscape, Illustrator & Corel Draw. You will need the Silhouette software for Print & Cut. The new Silhouette Studio is amazing. I really enjoy using it.
Silhouette does have a more accurate cutting formula than the Cricut. This is because of the motor. Also it uses different mathematical formulations.
One of the scrap queens Kristina Werner has a comparison right here: http://www.kwernerdesign.com/blog/?p=2841
As you can see it's a big difference.
These are the price points of these machines.
Over at the Cricut board none of us are willing to pay more than:
$250 for an Expressions
$100 for a Baby Bug or Create
$20 for cartridges
Silhouette runs deals quite frequently. I saw a deal over at 2Peas forum for $159. You will usually pay around $250 for this machine.
The blades & mats are very expensive for the Silhouette. You can't just run out and get them at your local craft store like the Cricut. They need to be ordered. Blades run around $20. Mats are the same amount. Don't faint yet, lol.
The blades last a long time. I have had my blade for over a year now. It's still strong. I used my Wishblade blade for over three years. If you take proper care of them. New mats can be made with quilting template or plastic chopping mats.
Like I said there is a lot of info on these machines, lol. I only covered 1%.
Which is the better machine? That is a hard question to answer. It all depends on your needs. I do frequent Print & Cut so I love my Silhouette. Cricut, I love it for the cartridges. I have a BossKut Gazelle and she is my main cutter for my designs, heavier materials and mixed media work.
I know I wrote a book. I do want to add one point. If you are looking to buy the Cricut only to use it with MTC, SCAL or FairyCut then please reconsider. I have highly recommended the Cricut in the past. I want to say this. The Cricut is a cartridge based machine. It is a mid-level machine. There are other cutter's on the market that are better for your own design work and downloading SVG's. You won't pay much more and have higher quality.
I hope I helped you a little.