Voting for the decathlon bike because it's cheap as hell and I had one for a couple of months.

If you are planning on riding "easier" stuff then it will work pretty well for what you want to do with it. There is nothing wrong with riding easier stuff, it's how I started riding bikes again even though now I seem to be semi-permanently injured.

If you want to throw more than $100 at a bike, look at buying used and you can get some awesome deals. It sounds like you want to dip your toe in to riding though so I still suggest the decathlon bike.

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I also feel that sometimes people need to be reminded: This is a "specialist" sub where we are all super focused on one hobby and look to the pros. Yes I agree the majority of walmart bike-shaped-objects are a pile of trash but for OPs use case & the current discount to be had on that ST100 I definitely think it is worth it!