First off, you're talking apple and oranges. 9006 and 9006XS are basically the same but are either straight or angled ends. These are halogen bulb types which are correct for stock housings. you can fit 9006's in but they are tight.
HID's are different. HID's are HID's are HID's. No difference except in kelvin ratings. Anything above 5000k you loose light output. The difference in so called plug and play kits are they are rebased HID bulbs so they can sell them to joe public thinking they can just slap them in and they'll have great lighting. This is far from the truth. (and is why a lot will be labeled "offroad use only" - to escape liability and misuse which is an oximoron as they are designed so they will obviously be mis-used) and in fact you have much worse lighting. as so much light is scattering everywhere (glare) but where you need it. Ahead of you on the road. All HID bulbs are straight. True HID bulbs are DS2, DS1, etc. as well as they do not use amp connectors. They will never fit any kind of halogen or stock light housings for obvious reasons. As said before, putting such HID's into halogen housings, you blind everyone as the way each work and reflect are different. You wouldn't put diesel in your jeep to run would you? It's a gas is it not? Same type of thing. Yes, HID's can give you much better lighting, but they need to be done properly to achieve that. If not, your lighting suffers and so do other drivers. Unfortunately it happens far to much. Even to the point I see mini-vans doing it all the time now.
Yes, there are 35w and 55w and even 100w ballasts available. 35w are more than adequate, and of course, require less power to run. Depending on application, heat from 55w can also be a factor.
The beatings will continue until attitudes improve!
HID Quad bi-xenon + foglight retrofit