They're from the same stable. Manstore tends to have the more flamboyant materials and designs of the two but they are both definitely fun! I even enjoy wearing the Manstore cheeky style briefs they do. Quality wise I think they are worth the extra premium you pay for them. I've owned a purple velvet Tower String for well over 6 years and the colour is as vibrant as when I first bought it and it hasn't lost its elasticity either. I can't say the same sadly for one of my staple Bruno Banani thongs, which I wore weekly, and bought around the same time.
They tend to be on the more generous side of things size wise these days. Styles like the Manstore M601 Rainbow String or M103 Tower String are ideal for just about everything.
I've bought a number of Olaf Benz thongs the last 2 years. I agree with Casper that I think the 'mini strings' are mostly the same cut of thong but there is a difference between the fabrics they use. They are great for wearing to work under trousers or under jeans. My most recent style, which I love, is the 1804 String Tanga.
https://www.deadgoodundies.com/olaf-benz-red-1804-string-tangaIt's definitely more one for play.