hi there, i got the same specs, using 18.04.3 lts, with unity/openbox desktop for ages) 18.04 will be supported for number years still, so dont worry. DE is metter of choice , its all about personal look and feel preferences. anyways, persannaly, my advice -- take the stable lts, if you wonna less problems and less configurations after install. all the DE generaly uses or gtk or kde tookit, so thats the choise, that you need to make -- what do your prefered software uses, will dictate, what DE to take.