Gee, you guys are just full of good questions. How come none of you ever sat in my class? Let me try to hit some of the salient points.
I've never seen an iPhone 4S, so I have no idea about the external metal band on the left side. If metal gets in the way of the antenna, it could be a real problem. But, we don't really know where the actual antenna is.
Think about the normal way that you are holding a GPS receiver or a smart phone when you are looking at the screen. Think about where your head is, or if it is shadowing the antenna. As a general rule, your head is blocking at least some of the view of the sky toward the horizon, but that should have relatively little effect when it is within 10 degrees of the horizon (because satellite signals are routinely masked when they get below 10 degrees). If the device is almost horizontal, and your head and shoulders are directly above the antenna, then you are probably causing a problem, either small or moderate. Most devices can get a fairly decent accuracy just by looking at the remaining satellite signals.
"MDM6610"? I have no idea what that is.
GPS+Glonass support. That is an interesting question. The U.S. put up its GPS satellite system first. The Russians followed with Glonass. For some serious periods of time, the Russians could not afford proper maintenance on its system, so accuracy was seriously affected. At the last that I heard, they had pulled it together again. So, if you listen to one group of people, GPS+Glonass is superior. If you listen to another group of people, that is all marketing hogwash. I've done talks alongside some of the biggest GPS experts in the U.S., and they will tell you that it is _mostly_ hogwash and you don't need Glonass for anything unless your first name is Ivan. Theoretically, the U.S. can drop the ball and the Russians can keep their Glonass cooking, or vice versa. Personally, I have my own views. Galileo is one other system that I am not putting my money on.
WAAS was kind of another big buzzword a few years ago. Yes, it does help a tiny bit in accuracy. Is it worth the trouble? I don't know. I do not currently use any WAAS-enabled receiver, although I have had my hands on a few dozen. One problem here is that most normal receivers and WAAS-enabled receivers have other small accuracy errors going on, so whether you get 1-meter or 10-meter accuracy from WAAS is kind of a moot point. One group of people is striving for the 1-meter accuracy, because then they can use it as a primary navigation system for human-less driven cars. That is getting way outside the bounds of what a backpacker needs to find his way along the trail and out of the woods, in my opinion.
If an iPhone drops its satellite lock, that could mean any of a dozen things. I've traveled back and forth several times across the U.S. when somebody complained of just such a problem. It could be a problem in that one phone device, something about the way the user is holding it, multipath interference, antenna shadowing, or high power interference from a microwave source. Been there. Done that.
Cleanliness of tracks. That is generally a software issue. Look up Kalman Filtering. Also, the more that you shadow the antenna, the worse this problem becomes.
Some smart phones use cellular augmentation to GPS. Generally, they pick up terrestrial cell tower signals slightly before they have a GPS satellite signal lock. That is kind of a temporary thing, in my opinion. Besides, most backpackers are so far out in the boonies that there is no cell tower signal.
If a Garmin Vista HCX takes forever to acquire your location, I could believe that. Most GPS receivers have a Time To First Fix (hot), Time To First Fix (warm), and Time To First Fix (cold). TTFF cold can be lengthy, and it depends on how much of the satellite ephemeris data is still in memory and how old the data is. So, before you go off into the woods, as you are getting your backpack out of the car, first turn on your GPS receiver and place it in a place with good view of the sky, such as on the hood of your car. Leave it running there for some minutes, up to 15, and I can guarantee that it will work better than if you just wait until you are out in the woods and you start it up. Some receivers get a bad component that holds that data intact in memory, so they have to wait until long after start up before they show good results. If you are deep in the woods and moving when you try a cold start up, you will run into what we call the picket fence problem, and your accuracy may be poor for a while. If you have the device deep within a car (underneath the metal roof), then you aren't making it easy for accuracy.
If you want to see something really cool, get yourself a Mil-Spec GPS receiver with the crypto key installed. Bring along your security clearance for that.