I say both. I'm in basically coastal NH.
There is a guy here w/ a ~5 year old truck that looks really great underneath, he's in NY (not sure what part). He washes it very regularly - don't know what schedule, maybe he comes through.
At the same time, you may not get it all off, you may park where it's wet in the summer. You may not be able to keep the salt of it for any period of time due to the drive from where you wash it to where you park it. You may not be able to get to washing it on the schedule needed. Rust depends on many things.
No I do not think the "better sort of paint" they use on the frame is all that great versus some other paint. I do believe our bodies and/or undercarriages may be galvanized, then painted - that does help a lot more than paint alone. They are certainly not impervious though. Also keep in mind, although the frames may have a decent paint on them, a lot of the parts do not - ie the paint on the rear end I found is very thin and when removing a sticker prior to undercoating I found it is easily removed by any solvent (ie mineral spirits, alcohol) - no idea WTF sort of paint that is but basically a terribly cheap and surely ineffective barrier.
BTW check your frame and undercarriage for paint defects/damage before you have anyone undercoat it - my truck that had 3 miles on it, the dealership, manufacturing, or transport, damaged the paint on the frame in 4 symmetrical places from lifting. Small defects, a dime size here or there, but it already had surface rust on those spots - I sanded the rust away, cleaned, touched up w/ primer and paint. Yes is probably futile as anyone without rubber pads on their lift points will do it again, but may as well start right.. Had a couple spots like that from when they were building the truck too on the underside/body, however those didn't rust - I just similarly touched up.
What undercoating do they offer? There are a bunch.
I'd read up on different ones and pick your poison, but you are going to find reviews that go from people swear by one brand, to people claim it actually causes corrosion
- I think for sure, doing it early is better than later. Here's what I learned, I just undercoated mine, myself - kit was about $300 materials, $300 gun (total $650/shipped) through Rovers North for Waxoyl, took about 16 hours labor - however it's a slower job on ramps, having the sun set on you. So w/ a lift, experience, if not being as meticulous, maybe less time. But $600 to me sounds like a half ass job.
Here's kinda what I found in research:
Lanolin based products: Fluid Film, Woolwax - these stay soft/wet, permanently, are less resistant to washing, require re-application more often, may actually be washed off w/ an undercarriage wash. Application is really simple.
Non-Lanolin Wax based products: Waxoyl, Cosmoline makes one, "Boss Wax" (not available for sale, is a system by a company called NHOU) - these can be softer/wet or curing to different degrees. Some have a very much solvent base and will be more dry, some are more of a base that does not dry at all. I'll explain the Waxoyl system below. These tend to be longer lived, can survive undercarriage washes, possibly even powerwashing depending which product exactly. Application of the stuff that cures requires better attention to cleaning, however is fairly simple.
Rubber based products - Ziebart, others - never seen it personally, but seems to be more akin to the undercoating in a can we probably have all seen, applied professionally. These are long lived of course - the downside being essentially it's a paint, so to touch it up you are talking a process akin to painting - difficult.
I bought the Waxoyl story - there are lots of places that apply it around here and I like that what's on the exterior isn't actually a rubber paint substance, but also cures to an extent. A couple calls to shops and I found nobody wants to return my call (or work), so I decided to DIY and bought that kit..
Waxoyl, it's a 2 step process. It comes in cans or by the 1.3 gallon jug and you use a spray gun - really I think any complete application will involve the spray gun, you kinda need it, though a an aerosol can of each product is handy for certain things and for touch up. There are 2 products involved, 120-4 & Hardwax. Prep is basic - wash the vehicle, remove loose rust if applicable. They say you can apply it to a wet vehicle and it displaces water - I dried my underside first.
The 1st step, 120-4, is clear and stays relatively wet for life - it is fogged into places that you can't see using a small hose that has a special fogging fitting on the end, hooked up to a spray gun. It goes up in the frame, in the rocker panels, in the fenders, doors, tailgate, hood, wherever (I skipped the hood & doors, didn't feel it'd be that helpful or warranted).
The second step, Hardwax, which is black and cures to a point where it's got the consistency of cold butter, is sprayed everywhere you can see - everywhere that gets abrasion from the road, idea is basically so it holds up to driving and washing.
