I'll be talking 1/35 only since I don't know about 1/48 scale.
For Hueys the only is the Dragon/Panda UH-1N that you already have. To improve this model you'll need Cobra Company's UH-1N Fleet Upgrade Set. If you're up for a gunship you'll also consider Cobra Co.'s UH-1N Weapon Set. There's no specific PE for this model, but you could use parts of Eduard's UH-1C interior and exterior set. The only aftermarket available decal sheet is MAW's "November Hueys".
Now to the Cobra (BTW I'm currently building two of these). The only models out there are MRC's AH-1W (it's been reboxed by Academy and Italeri) and Academy's AH1-W NTS; even though latter appears to be a rebox of the first with only a new decal sheet and no NTS. For this you'll have to get Cobra Company's NTS Upgrade set. Eduard has three PE sets: interior, exterior and armament. Two Bobs has two decal sheets for the Super Cobra: "Whiskeys in the Mix" and "Sharkmouth Vipers".
Currently the Marines are starting to use the UH-1Y and the AH-1Z, but there are neither kits, not upgrade sets available for these two aircraft.
HTH