definite improvement - excellent storyline and better options for planes, weapons and customization than acecombatX
AC joint assault improves on the winning formula set in X by allowing you to customize your planes payload so you aren't always stuck with the standard missiles but now you can mix it up with 2 special payloads instead. there are more choices of planes and specific weapons go on specific models only but you also get to buy and apply wing, amament, armor, etc improvements to your plane if applicable (not everything will fit on everything). Much better field of choices this time around.
the graphics are improved but considering X was an eyepopper i'm not too surprised from one of namco (sorry i mean BANDAI-namco)'s flagship titles. there are now jutting rectangular buildings when you get to cityscapes and pyramids in the desert, etc... i AM surprised the psp is still relevant after 5 years and still the games look darn good.
the sound quality of the music, sound effects and voicework are typical namco candy, always never even a factor. everything sounds just right or as you might expect them to sound. the background music inflight is exciting or forboding when it means to be.
the gameplay is more of the same carried over from X; silky smooth controls and a variable difficulty setting for novices to real aces! level design is much improved and you get twists midair on what's happening so you've gotta keep your attention in the game which translates to a better experience.
so what are you waiting for?
see you in the skies, ace.