Definatley one of the most asked questions on here. Basically its personal prefference. If you want digital, go with SPA. If you want Mechanical, the 2 best companies by far are VDO and Autometer. They are the highest quality you will find and the most options. Autometer currently has 3 gauge sizes, 4 lighting colors, and EIGHT different styles, Sport Comp, Pro Comp, Z Series, Phantom, Ultra Lite, Carbon Fiber, Sport Comp Silver, Auto Gage (Low end), and soon LUNAR series which will be cool as hell when there done. They offer boost gauges in 0-20psi, 0-35psi, 0-50psi and 0-60psi, and they offer Electric, Mechanical, and Mechanical Isolated pressure gauges. They are my favorite based on quality, customer service and hands on expierience. They are also used by Virtually ALL of NASCAR, NHRA, IHRA, IHBA, CART, and lots more. VDO's are really nice too, just not as many options.
As for SPA's, i like the "2 gauges in one" idea, but with a triple pillar, i can monitor Boost, EGT and Trans Temp with my Autometers. Thats all you need to monitor. Fuel pressure is good too, but i dont need it in the pillar in my line of sight, i mounted mine below the dash so i can glance at it every once in while, i dont need to see it all the time. With a 5 or 6 speed trannie, you dont need a trans temp gauge and an oil temp gauge is really unnessecary. Also, SPA's are WAY too expensive for me, a complete set of three of there gauges and senders is well over $500. 00. They are also not dimmable. I got all my Autometer gauges, senders and the pillar for about $300. 00. Also, personally i dont like how they are always jumping around. Just my preference, pick what you like.