To be honest and I'll start by saying I have never built a S-27 or MFH kit, but it is really hard to compare the three against each other. All of these manufactures fill a nitch in the hobby.
Tamiya kits practically build themselves. There are virtually no fitment issues with these kits because the are so well engineered. Add to that the fact that they are produced in large numbers making the price more reasonable for the average modeler and it is no wonder why Tamiya has the reputation that they do.
Studio 27 is by far the most advanced sans MFH of the "Garage" manufactures. As Doc pointed out they make many subjects that you just can not get from any of the plastic model companies. How ever it is important to keep in mind that they are a "Garage" company and thus produce kits in a different way to that of Tamiya. The runs of kits are much lower in numbers and the prices therefore are higher. They do require extra work in fitment etc because many of the original parts are hand made and not cast in an machined from aluminum injection mold like a plastic kit is.
MFH (Model Factory Hiro) Is the premier "Garage" manufacture. they incorporate many state of the art manufacturing processes such as 3D cad design of parts and 3D printing of prototype parts. Some of there latest 1/20th scale F1 kits have been compared to Tamiya's 1/12th scale line of kits in the area of detail. However, it is also important to remember that even with all this technology behind them MFH still suffers on occasion from fitment issues like any other "Garage" kit company. And like any other kit company, plastic ones included, they also simply flat out make mistakes.
If you feel like you are not challenged by the average plastic model anymore then I would suggest trying a S-27 kit. One thing you should realize though, and I think this can be said for all companies in the industry, the newer kits will most always be of a higher standard then the older kits in a companies range of choices. So if your going to try one of the other brands better to pick a newer release then an older one.
Remember, I have never built a S-27 or MFH kit but I do own almost all of them so take what I'm telling you at face value.
