What to get? How do I know whether what I'm looking at is a BSO? (Bicycle Shaped Object); what should I do about the multiple chainrings?
That Munich &quot;21-speed&quot; is really a 7-speed. It doesn't have a triple chainring or front derailleur (and doesn't seem to have a planetary-gear rear hub). Not sure why they call it that.
Both bikes seem pretty comparable. Decathlon is well-known in Europe as selling cheap but decent products. I've never heard of Mach City. If a brand's reputation is worth anything, that's something to consider.
One of the hallmarks of terrible BSOs that I've seen in the USA is thin stamped dropouts with tubes that are pinched onto them (see below). Neither the Mach City nor the Decathlon bike seem have this kind of construction. Another hallmark is the one-piece crank.