JLT is perceived to be downmarket compared to the Marina purely because the Marina is closer to the beach and has lot of pretty boats. The buildings themselves, there's not much more difference. The Marina has some better quality buildings, but you'll pay more for that. Generally, cheap apartments in the Marina are turd and you'd get more bang for your buck in JLT.
Traffic in JLT can be bad, but it depends on where you live. There's a new road that goes to the far end of JLT from the Marina interchange that bypasses most of the problems. If you're in Clusters D/E, then you will have zero issues with traffic. Cluster C would be fine too in terms of Metro proximity.
You could easily have a relaxing walk to the beach from those clusters, I often park my car in Marina Mall and walk to JBR, add in a trip across SZR by Metro bridge, and it's not too bad. You wouldn't be doing it in summer though. There's plenty of taxis in JLT and a journey to the beach wouldn't be much more than 15 Dhs and 5 minutes of your time.
There's not too many bars in JLT, McGettigans in the Bonnington Hotel is very popular though and always bouncing. Marina is only a short taxi ride away though. Generally, you'd likely be taking taxis to bars in the Marina if you lived in the Marina anyway. There are plenty of restaurants in JLT around the lakes though, massive selection and more and more opening every week.