Sam's right, there are a ton of different brand names that were widely sold and used. Dixco's are OK, but were indeed a "cheaper" Sun/SW knockoff, usually made of plastic instead of metal, and were less expensive. RAC also falls into this category... The positive thing about them are that even today, they are usally cheaper to buy, and read RPM's just fine, as well as look cool. I have even seen some no-name brand vintage tachs that were cool. The key is to make sure its of the same vintage as the car, not only does that mean not too new, but also not too old. You really can't go wrong with either of your choices, just a matter of personal opinion, and finding a nice tach. BTW, the above pic Erik posted is indeed the 69 Phase III Motion car. Tach is a Sun ST-802, a transistorized 8,000 version, very popular and widely used during the late 60's & early 70's.
