I am going to do my best, to compare them by paper.
The Logitech G27, when it launched, people knew that it would be quite the wheel, given how good the G25 was. The wheel would be good for modding, and good for any realistic cockpit projects. The G27 would be smoother than the G25, have stronger force feedback, and have pedals modified ever so slightly, to feel much better. The pedal was just what the customers wanted.
These days the wheel is website listed at 299.99, with free shipping, and what you get is the wheel itself, with its dual motor, helical geared force feedback, paddles made of stainless steel, shifter lights, a rim made up of real leather, and tried and true pedals, with adjustable positions for the pedal faces. The shifter comes as a slight downgrade from the G25, losing the sequential function. This wheel also has 4 different adjustments for steering degrees in game.
Fanatec has a different approach to selling a wheel. They allow you to either buy the wheel standalone, and use the good old Logitech pedals, with an adapter of course, or you can go ahead and buy some pedals, and if you must, a shifter set as well, but this can get expensive, so they have come out with bundles. The two bundles that are most mentionable here, with only PC and PS3 in mind, the Porsche 911 GT3 RS V2 Value Pack ($199.95), and its XL variant ($259.95) both prices before shipping. The wheel you get for both is the Porsche 911 GT3 RS V2 wheel, with its 550 sized motor powering a belt driven force feedback system, with two more motors for vibration feedback. The wheel is made up of "Automotive" grade materials. This can mean anything depending what countries these materials come from, with U.S. cars being quite cheaply made (bring the heat), and German cars, being made with the top notch. The wheel is adjustable with its built in menu, and can store up to 5 presets. The rim is hand stitched, much like the G27, but this one is with Alcantara leather. The angle of steering can be changed in increments of 10 degrees, and to whom it may concern, has a drift mode. The paddles that come with the wheel are aluminum, but there is an optional carbon fiber set. The base of the wheel is a nice GT3 RS colour, and the wheel has a ring at the top, for a bit more of a racing feel
The normal value pack comes with only the CSR pedals. Spring tension is adjustable, the spacing between the pedals is adjustable, the spring tension on the pedals is adjustable, and the angle of which the pedals sit is also adjustable. The pedals are longer than the G27 standard, and there is an option to buy an inversion kit, where you don't need anything else to mount them to.
The XL value pack has the CSR Elite pedal set, it is for the most part the same shape as the normal version, but this one has a lot more metal in it's construction, and also has a load cell in its brake pedal. This means that the brake pedal will have a much better feel, which can lead to faster lap times. The inversion kit also works on these pedals.
Included also with XL pack is a shifter set, and to metal beams to attach them to the wheel. In this set comes an H-pattern 6 speed shifter, with reverse, and the other being sequential, something that the G27 does not readily have. The shifters have similar build quality to that of the G27 one, but without the leather knob. These shifters can not be clamped to anything but the wheel with the metal beams, but they can be attached on either side.
These two options allow a buyer to better fit the price to their wallet, but although the prices are lower, the shipping will probably be a big factor for choosing between the Fanatec wheels and the Logitech wheels, with Fanatec shipping for myself (Canada) bringing the XL to about $420, which is about two slightly used G27ses This is mostly the fault of UPS, but I am sure the shipping is much lower for those below the border.
Summing it up. Logitech has been known in North America for making wheels that work well, and last a long time doing it. I have not personally had a long test run with a Fanatec wheel, but the wheel does feel great, especially for it's $250 price tag, and has plenty of features that I, and probably many others have never even used. People do say the FFB is too high for their liking, but it can be toned down, allowing for perhaps a longer life of the wheel. If you can, try to give each wheel a test with GT games, just to see how you like them, but in the end, price is the factor for deciding the wheel. The wheels are close enough that if you like Porsche, well than you know which one to get.