Which Straumann overdenture attachment to use?

I use locators. They're available for every implant system. That way I don't have a bunch of different nylon replacement parts for different systems; just one attachment system.

Cendres & Metaux (the dalbo guys) have made there own version of a Locator type attachemnt that used PEEK instead of plastic inserts - it is superb, but i f you have 2 implants and have them paraallel to each other then nothing (including Locator) beats the Dalbo ball attachment system - yes people complain about ball abutments buts its all down to getting the implants no more than 10 degrees divergent. I have cases that are now at 6 years with no ball wear under 5x's magnification and no prostho issues. Look at the C+M version of Locator as it really is a nice bit of kit

It all comes down to the axis of rotation, the position and number of implants and what the overdenture opposes in the other arch. Yes, parallelism counts. I have used the Straumann Dalla bona style round ball abutment with success over the years and used their blue plastic handled lamellae tightener as needed for the matrices to be tightened in the overdenture on recall. Their success is dependent on preventing the matrices from rotating at the time of chairside pick up in the overdenture's intaglio. I used to use rope wax around the gingival half of the abutment clinically to prevent Acrylic Solder/ProTech pink pick-up methyl methacrylate from locking into the undercut of the ball abutment emerging from the fixture with great success. Lab processing can be done with analogs. I prefer chairside attachment with a conservative amount of pick-up acrylic. It does not apply to your question, but the only ball system that didn't rotate and pivot appreciably, IMHO, was the large diameter round abutment ball with the tan nylon housings and o-rings from Nobel. Unfortunately they were only designed for use with their 3.75 external hex Branemark implant system. In my experience they do not readily pivot and a periodic o-ring change keeps patients more than satisfied. The Dallabona system 'does' rotate. 22 years in Prosth I default to the Locator abutments or cast, milled telescopic abutment/over copings. ERAs are another popular option - in my hands they do not perform as well and require more maintenance. The Locator system is superior in: its design (tipped hat to Dr. Bob Vogel); less rotation around a fulcrum even if there are only 2 implants; and infrequent patient returns with loosening overdentures. With many retentive color choices in the nylon replacements you can meet the whims of weak patients who need their overdenture easier to remove (blue) and those patients who never feel like they are tight enough (clear). Of course, your mileage may vary.