I run both a GoPro Hero 2 and HLT recording iPhone 4S video. They each have their strengths and weaknesses depending on what you want to do: (make sure you bump up the resolution on the YouTube videos below)
HLT Pro with iPhone 4S video:
The up-side: The iPhone records a fine movie. The convenience of using HLT to overlay data onto it's own video is awesome. ZERO effort is required to make a nice watchable video with great data info overlaid. (I use the Emprum UltiMate GPS add-on. It makes HLT EXTREMELY accurate. I compare it's times to video and a transponder and it's within 3/100th of a second at the worst.) For instant gratification, nothing beats HLT with it's data plus video overlay. And it looks good enough that you're not feeling lame when you show it to someone. The colors are not as deep as a GoPro, but I bet if someone spent just a little time on color-correction, it would be great.
The down-side: Mounting options are limited in terms of where you want to put an iPhone. I would never put my iPhone outside the car at 150MPH, so your video viewing vantages are limited with just the iPhone. You can't choose field of views with the iPhone---no wide angle options for more field-of-view capture. If HLT crashes---and it does sometimes---you lose video and data. If the iPhone shuts down as a result of overheating because it's sitting in the sun too much----which happens when it sits in a hot window in the sun for long periods, all video and data capturing opportunities are lost. The iPhone 4S rips through battery life like a hungry dinosaur----especially when using an external GPS, HLT and recording video. It's easy to run out of battery if not careful, and then again, data and video are lost (using the micro-USB charging port on the Emprum has been flaky lately in the car…..plugged in but not charging….not sure why). iPhone can only record in 1080p. There's no way to reduce the resolution while recording. This makes for huge files.
Here's HLT Pro recording video on an iPhone 4S at Auto Club Speedway:
GoPro Hero 2:
The up-side: Really sharp video. Different field of views available. I shoot in 720 resolution to keep file sizes manageable. Still looks great. Whereas I don't put my iPhone outside the car, my GoPro LIVES outside the car. And let me tell ya, it gets peppered with all kinds of stuff from the track. I'm already on my second lens cover because the first one was pock-marked and pitted to hell from all the junk flying around. The GoPro's batteries can be replaced quickly if they die with ready-to-go spares. The mounting options are limitless---bumper, roof, any windows or side panels, trunk. My favorite view is suction-cupped to my lower front bumper. (At the bottom, you can see a pic of the GoPro suctioned to my bumper.) if you want to impress someone with your "speed", show them a bumper camera……..windshield camera angles look like you're noodling around going to the supermarket no matter how fast you go (the two videos I'm linking to compare the same lap----one from the GoPro on my bumper, and one from the windshield and the iPhone. Which "looks" faster? The one lower to the ground ALWAYS looks faster. But you probably wouldn't want to put your iPhone down there.) Anyway, you can put the GoPro pretty much anywhere inside or outside the car and it's fine (if you put it too far back inside the car and the car is dark, it does struggle to keep from washing out the outside---no matter what mode you put it in). The GoPro is immune to the sun and heat---never shuts down from that. It's been used on several 100+ degree track days, and it's had no problems. The new WiFi BacPac with Wifi Remote makes starting and stopping videos convenient. It will be cool to see the video streaming app to the iPhone.
Since GoPro makes so many mounts, the GoPro is very useful for other things besides recording car videos. I bought the case for taking sharp, well-focused videos underwater----so awesome!! (The original case is waterproof, but doesn't focus properly underwater.) The video while swimming in a pool or snorkeling is amazing!! And then I have mounts for my bicycle, a head mount, a chest mount……the GoPro is amazingly versatile.
The down-side: If you're only Mac-based, doing a data overlay onto a GoPro video is tough. The programs that do it after-the-fact are all Windows-based. I have Fusion with Windows XP on my Mac, so I bought Dashware. But doing an overlay is a LOT of work. You have to find the lap you want, save it, copy it over to the PC, export the data from that lap to Dashware format, import that to the PC, learn how to use Dashware, I end up making my own gauges because the preset ones never do what I want, then you have to hand-synchronize the data with the video which is completely trial and error. Watching the video is tough because you're not using a native Windows computer, so the emulator chokes on the video which makes it even harder to synchronize the data. Then you have to export it, which takes awhile to render, and then get it back off the pc to the Mac……..sheeesh……HLT makes that a 1-button-click process….And if you want to do it over multiple laps, then you have to hand-synchronize each lap's data……HLT won't simply export just three laps of your choosing together……ugh…..it's not easy.
If you have the new WiFi BacPac, then you have at least THREE batteries you need to manage---the camera battery, the BacPac's battery, and the WiFi Remote's battery…….if you leave one of them on too long and drain it, then you're hosed…….and it's easy to forget to turn off the BacPac……oh, and the WiFi remote uses a bizarre custom charging connector, so you have to make sure you have that stupid little cable with you or, again, you're hosed if it runs out---that battery cannot be replaced with a fresh one. But at least if the GoPro battery dies, you can have fresh spares ready to go.
Here's the GoPro video of the same Auto Club Speedway lap from the front bumper. I didn't bother doing an overlay:
Here's a GoPro video from Big Willow with a simple overlay from Dashware:
Interestingly, (I've never done this) there's a way to have HLT overlay it's data on video that you import to the iPhone, which I guess could simplify things a little bit, but it would still be a bunch of steps. At least you wouldn't have to bother with a pc and a whole new app.
Below is where I usually put my GoPro on my bumper.