Depends where You live, i also wanted to choose between those two, in the end i realized that integrated filter wheel with 5 slots is not very good thing, only bonus for Atik 9 is cooling, 490 has only -25 below ambient (mine goes -30 with no problem ). So if you live in a place where you have mild weather i suggest 490 and filter wheel with 7 slots.
Important thing is to achieve -20 degrees while imaging - no hot pixels,very very very low noise, at -15 degrees You will see hot pixels at long exposure times (20min +)
So if You live in a hot place then get Atik9.You will need to change filters from narrowband to RGB total set is 7 filters You have 5 slots only, in 5 slots You can put LRGBHalpha filters for galaxy imaging its nice, problems are with narrowband, using Ha, S2, O3 filters + you can put in two of color filters, sad not 3 to make nice colors for stars.
Otherwise take Atik 490 + Atik filterwheel with 7 slots, remember goal is to achieve -20 during night, should not be a problem if live far from equator (short nights in summer so sumer counts out) how hard is to swap filters in Atik 9 i have no clue, need to ask someone who uses this camera.