Ignoring soft-modding because that would be cheating since Brawl offers Project M and custom mods
Super Mario Bros. Melee
+Better game balance (major plus)
+Better items
+Better self destruct system
+No cuts from the preceding game
+Better AI
+Fun stages
+Truer to the representative game's source material
-Archaic method of unlocking trophies
-Even worse method of unlocking some characters
-L/R button sensitive shields
-No backwards grabbing edge
-Worse Ice Climbers
-L-canceling
Super Smash Bros. Brawl
+My Music
+Great stages
+More stuff to collect (trophies return along with stickers)
+Stage Builder (not the best but better than nothing)
+Coin Shooter
+Tether grabbing improved
+Boss battles (not the mode, in general)
+Attempted to make a more in-depth story mode
+Some great items
+Assist Trophies are always fun (except for the bad ones but those are expected any how). Same with Smart Bombs
+More characters
+Third party characters
-Atrocious game balance
-Heavy emphasis on defensive combat
-Though it has great stages, it has utterly atrocious stages too
-Has good items, has atrocious items
-Pity Final Smashes
-Characters having only a tether recovery as a recovery option
-Though the attempt at a better story mode is commended, it's executed badly
-Doesn't follow the game's source material (Mushroomy Kingdom is NOT representative of Mario, nor is Luigi's Final Smash!!)
-Terrible Pokemon characters
-Questionable cuts
-Doesn't fix some issues Melee has (clone Ganondorf, clone Link)
-Multiple air-dodges
-Though it has great stages, it has way too much sh-- stages
-TRIPPING
Brawl's flaws are more glaring than Melee so I'd go with Melee