I'm not terribly familiar with Breitling models in the early 1970's, or any era for that matter, but the one thing you have to be concerned about with Breitling is that they often used some lesser used chronographs, (like Venus, some of the more economy minded Valjoux 773x, which might not be the easiest to keep, maintain or repair. This might not be much of an issue now but in future it may become one.
The Omega's in the 1970's were very strong competition for Heuer. In the spirit of full disclosure I own quite a few of them. As Omega was strong competition for Heuer you can be assured their products were at the very least comparable in quality.
The three main movements used in tonneau shaped case in the 1970's were the c.861 (same as the moonwatch, Lemania c.873), c.1040 (Lemania 134x), and the c.1045 (Lemania 5100), if you go back into the 1960's they also used the c.321. Parts and servicing for all of these movements are currently fairly easy to get, although if you need service for anything other than the c.861 movement it has to go to Switzerland because Swatch Group US won't touch them (not that anyone sane would want Swatch Group US to touch them). However, most local watchmakers have no problems servicing them.
Omega made some very beautiful and rugged watches in this epoch... Among them Seamaster's: 145.016, 145.019, 145.020, (Darth Vader/Anakin Skywalker) 145.023, 145.024, 145.029 (all of the 145.xxx's are c.861's), and the 176.007 which uses the c.1040 automatic movement. Among the Speedmasters, if you're talking about 1970's models, you should consider the Mark II (2nd version) (c.861) and Mark IV (c.1040), the following c.1045 Day-Dates: 176.012, the 176.0014/176.0017, the 176.0015/176.0016, the Speedmaster 125. If you want to expand the scope of your search to the early 1980's you should consider the Mark V, and all of the Teutonic c.861's... Omega also made a line of Tuning Fork watches, The Speedsonic, while it's most deluxe model, the Lobstertail, isn't by any stretch of the imagination a Tonneau cased watch, the rest of the line is. And that line is best covered by LesZ's outstanding article on the topic. Les even includes a movie of the 'Sonic in action! Additionally, if you which to expand your interest to non-Tonneau shaped watches, there is the Flightmaster line and additonal Seamaster's to consider.
The models that Heuer produced in the 1970's are best known by the folks around here, so I shouldn't need to cover them in too much depth. However, in addition to the Autavia, examples of the Calculator, Montreal, Skipper are worthy of consideration, as is the Pasadena and 1000/510. series.
The one area where Heuer and Omega competed against one another head to head in this epoch was the Speedmaster c.1045's and the Heuer 510.xxx Series. In this head to head matchup the Omega's typically have a little more "executive" styled cases compared to Heuer's Sportier look, but the advantage that Heuer has over the Omega (and remember I'm a big fan of the Omega's) is the Heuer's typically have easier to read dials than their Lemania 5100 powered Omega counterpart's. The real problem with the Pasadena and Lemania 5100 powered 510.xxx series is that they are getting very VERY difficult to fine on the market these days. If you see one you want, pick it up, you may not get many other chances.
