Spring or not, it's the antenna length that makes it resonate at the intended frequencies. The spring is just there for a level of shock absorption.
On a tall vehicle, the spring might be helpful for tree branch strikes, but that's about it. Usually the springs are pretty stiff, so they don't really help a whole bunch. A thin whip will fold over almost as well, however it may take a permanent bend.
Whip diameter does play into useable bandwidth, but on a 1/4 wave mobile antenna, it doesn't make that much a difference. I've put these on $20,000 analyzers and you'd be hard pressed to make that argument when you look at the plots.
On my work and personal trucks, I've got a VHF quarter wave mounted in the center of the truck roof. Looking at "usable" SWR, no issues from 144 to 170, doesn't even touch the 2.0:1 point. Stays below 1.75 or so across the band.
Of course it does matter when the antenna is cut for in the first place. If you are using an antenna that is cut for 160MHz it's going to do better closer to that. On my personal truck, I've centered mine around 150MHz. Works fine for my amateur radio use as well as our work stuff up around 159MHz. On my work truck, I center the tuning closer to 158. Still useable on amateur 2 meters, but also suitable for the fed interoperability frequencies in the upper 160mHz range.
As for which antenna design to use, there seems to be two trains of though on this:
1. Go cheap. $10 "chrome nut" antenna. If it gets damaged, just grab a new one and replace it.
2. Spend a bit more and not have to replace it as often.
Key here is that I've never felt a mobile antenna was not a consumable item. I figure if we get a few years out of them were doing pretty good. Seals rot, whips get bent, corrosion, etc will all take them over time.
We received 6 new police cars about 3 years ago. Installers had put on the $10 whips. Worked fine for a few years, then started getting a few complaints. Whip flexing from car washes, etc. have bent the whips over time and allowed water to seep down inside the seals to the mounts causing some corrosion. I've replaced them with Larsen NMO-Q's with the better sealed base. Will likely solve the water intrusion issue, but I still figure I'll be replacing them in a few years.
Important thing is to get quality antennas and tune them for the center of your band. Don't cut corners to save $5 per antenna. Whip diameter isn't that critical in a mobile install. Keep up with periodic maintenance. Make sure the installer actually puts them on an analyzer and checks them after the install is complete. Way to many installers just take the antenna out of the bag, screw it on and walk away.
