The balanced vs unbalanced argument is not always well understood; with the 1st stage, about the only unbalanced design you'll find is a basic piston like the scubapro MK2. With diaphraghm 1st stages, I don't think you can find an unbalanced one; could be wrong, but I've never heard of one. Anyhow, a balanced 1st stage provides a stable intermediate pressure regardless of the tank pressure, so the idea is that the very last breath of air out of a tank will feel the same as a full tank. An unbalanced 1st stage will start to drop in IP as the tank pressure falls below 500 PSI. In my MK2, this results in a SLIGHT increase in resistance in breathing that's noticable at about 300PSI and increases as the pressure gets lower. There are lots of divers (and dive shops!) that will say that this is a bad characteristic of a reg, maybe even dangerous. Personally, I think it's a safety feature; you'll still be able to get the air, the only difference is if, as a newbie, you forget to monitor your air supply (or your SPG fails) you'll get a "reminder" that you're low on gas. As a practical matter, any diver, especially a new diver, that's still submerged with only 300PSI probably needs that reminder to head for the surface.
Balanced 2nd stages require less spring pressure to remain closed and as such can be set to lighter resistance. This is a good thing, but it's important to remember that we're talking about subtleties that will almost certainly be lost on a brand new diver. Any decent quality reg that's in good adjustment is not going to feel like it's not delivering plenty of air. Balanced and unbalanced regs respond to depth in exactly the same way; if someone tells you unbalanced regs perform poorly at depth they're not telling you the truth. Now, the overall performance of a reg is tested more at depth, so high performance regs will deliver more air at depths. As an example I can tell you from experience that there is not much difference between my MK2 R190 and my MK15 G250 (which is a very high performance reg) at depths up to 130 ft, which is as deep as I've been with both. On direct comparison, you can tell the difference, but it's not at all like the MK2 is unsatisfactory at that depth.
I think regs like the SPMK2 and other basic designs get a bad rap from divers who often encounter them as rental regs; rental companies love them because they're cheap and durable. But, since rental companies often don't have their regs tuned well, divers who dive with them are often getting regs that don't perform up to par.
To be honest, there are so many good, reliable, inexpensive regs out there that if you're concerned about the balanced/unbalanced bit, you can choose from several balanced regs without spending much. What I would look for is a reg you can get serviced easily in your area and whereever you plan to dive. This tends to exclude some esoteric regs and favors the big companies that have world wide distribution; SP, aqualung, mares. Zeagle will not have the same international presence (like if you need a part in indonesia) but so many people speak so highly of the zeagle envoy that you'd want to take a look at it. Likewise, I'd recommend scubatoys as a good place to shop; those guys are great with service. But, you won't be able to buy aqualung or SP regs from them unless you go to the shop, although if I remember a while back they were closing out some MK2s and could sell them online.
It's a tough thing choosing a reg as a new diver; most people will just tell you to get what they dive, and it's tough to "try before you buy" except maybe in a pool with a rental reg. The good news is that you're probably not going to go wrong with any reg from one of the reputable companies.
