AS far as stationary bicycles, what are the particular reasons to use?

the UPRIGHT exercize bike opposed to the RECUMBANT exercize bike???? I am looking to purchase a cheap one......want to burn hip/tummy fat but have bad legs....Thanks......
Update: I mean, my knees are starting to get pissed off as ive jogged/sports/more over the years and im 45 and do some long walks.......(Saying not 'handicapped" just, think Id do better on a bike than say out running for awhile...THank you.
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