I stayed in both for extended periods of time many many times and Bangkok was love at first sight, though it get tiring after some time, KL not that much, but as for your hygiene comment I think they are pretty much on par, it's just that BKK is significantly bigger city so it might look a bit dirtier but in reality KL for its size is also pretty dirty despite being richer

though I would take nowadays Georgetown over both of them and maybe dare even to say I would prefer Kanchanaburi over Bangkok, their night market next to train station is amazing, went there for every dinner always when visiting, somehow I got tired of these bigger Asian cities and can enjoy the small as well (this coming from someone who lived in 20mil Beijing for years, so KL is pretty much village like most of European capitals by population)