Go UDM. Just isn't any question here.
The UDM is the PC with a little more power. -- I was curious about the power so went to the store and brought my UDM to test them on a panel.. They feel identical,.. the UDM just spins up a bit faster at the highest setting. (Also can handle a little more pressure before it stops its rotation due to motor torque) -- It is nothing ground breaking over the PC though. Either one is fine, and I would have just as easily taken a PC 74xx instead of the UDM.. I just chose UDM because of the package I Got.
You have a new car,.. with new paint, with minor, minor problems. You are on autopia -- Give this, you probably know how to take care of the car, and not marr it all up.
So you don't have a need for a Rotary,.. you won't need to be doing that level of correction. You have PLENTY of clear to work with, so no worries on taking too much off.
You don't need to spend a fortune to do a minor light polish on a brand new car, and likely not have to polish again for at LEAST 6 more months, and then the least agressive you can come across.
The only reason you would ever need a rotary with this car is if you had the 'perfect' scratch that could be buffed out with a rotary but not a RO -- This is not gonna happen.. It will either be too deep, or your RO will do just fine. Worst case you can wetsand it (which is better than all of the options anyhow).
I'm just trying to cater to your needs here,..
If you had a, say, 2002 Explorer.. With likely significant marring -- Then maybe , maybe a stronger machine would be warranted, but even then I doubt it.
Your new to the machine right? -- Learn on a PC/UDM (same design) --
TTWAGN says you are worried about more cutting power with the mention of the 4" pad kit -- and perhaps he is right, that is why you want the kit. (I think it might be more for getting into tighter spaces.. but who knows) -- You don't need more cutting power,.. You can do a rediculous amount of cutting with a UDM or PC with a Yellow pad and agressive polish. Way more than you would or should do.
As for pads.. You don't really need the 4 inch kit -- Though it is handy to have,.. comes with a 2 1/4" backing plate, and does help get into tighter areas, and , yes, gives you more cutting power should you need it at some point.
The flex is overpriced imho for what it does. If you need that kind of power, just get a rotary and save your money. I see absolutely no advantage of a Flex in design over PC/UDM types. It doesn't cost any significant amount more to produce the Flex over the Porter or Ultimate.. it is just priced up for hype and the 'you get what you pay for ideal' .
I'm not ragging on the Flex,.. it is a great machine -- Just as good as the PC/UDM, and more powerful I assume.. But I would just assume have a UDM and Rotary for less than a single Flex.
You get my point -- Buy what you feel comfortable with ,.. if you think you'll regret not having a Flex for your needs, you won't. If you think you'll get the PC/UDM and still want a Flex I have no clue,.. it won't be because what you have doesn't do the job though =)
Back on pads -- I would go with 5.5's to start with,.. don't even mess with the 4 inch kit. Just get 4-5 of each pad type of that size, and your favorite polishes.
You see,.. you'll spend a day, or a weekend polishing the car, and then the machine is put up and not used for half a year.. So no need to buy all the accessories .. unless you will be doing a car a week ,.. or running a detail bus. on the side
Now, I did do the following, just because I like to have more than I need.
Got UDM, got (LC/CSS) - 4 Yellows, 7 Oranges (had 4 free), 4 White, 2 Grey, 2 Blue. -- XMT 3, XMT 1, DWG, NXT 2.0, Meg. Carnuba, Glass Rest, Pad conditioner, pad cleaner, -- Complete 4" pad kit (backing plate, 4" Grey, 4" White, 4" Green, 4" Orange, 4" Yellow.. -- Came with Microfiber accessories as well) -- I also got some Menzerna polishes, and some proto-type polishes/glazes and wax sent to me by a new producer making a name for himself in the business to try out.
Tons more to add to that list, but that is the recent stuff with the Machine that I purchased.
I've used it all, but only once. I polished, but now the car is polished, perfect, nothing more to do to it, nor do I want to repolish just for the sake of doing it.
I just polished the windows with a restorer for water etchings (4" pads worked great here) -- Only time I used them.
The only things I use often now are MF towels, Car wash soap, NXT 2.0, DWG, Aero 303, Optimum Tire Shine, (I wax the wheels with NXT 2.0) , Leatherique, and a Vacuum cleaner
I don't bother using the machine for wax application or DWG -- It takes longer to get it out and using it to apply than it does by hand (for me) -- Plus the pads waste alot of the product (for me -- could just be my error ).
Get the machine and polishes you choose, and go over to the Detailing forum. Wash, clay, polish, seal, wax (glaze if you choose) the car.. and show us some before and after pictures. Show us what you can do to a brand new car.. You'll be surprised.
Some of the white polishes recommended and other glazes and such will make the white so reflective it will literally hurt your eyes in direct sunlight. No kidding!
Good luck,