The G4/5 is designed to do one thing well. That's to be a pager that does a great job of receiving P25 simulcast (yes you can get a dual band model that also does VHF or UHF). Again, it's a pager, not a scanner. It lacks things like lockout, TG hold, delay, and importantly, the ability to monitor multiple systems at once. Other than being able to switch to VHF or UHF (only one of those can be chosen to be added to a G5, not both), it's useless for things like airband as well as many other things. It will not monitor a Moto smartzone system (like MCSO). You can program a system to run on one knob setting that will "scan" all TG's that you want on that system, but you need to switch knob positions to hear one particular TG (lots of "tricks" to setting up knobs to accomplish what you want - see Unication forum). It is not a scanner, and it will not replace the need for a scanner for the serious hobbyist.
My short time with the SDS100 in the valley: It does properly demodulate LSM (simulcast), but the ability to resist interference and to pick up 700 MHz signals from a distance were for me not acceptable. Anywhere I went, the SDS100 was only able to pick up the simulcasts that it was sitting within (if those), and maybe one more simulcast in some cases. The G4 could pick up most or all systems, valley-wide, from those same locations. I can monitor Mesa Topaz from North Glendale with the G4 (30+ miles away to the nearest tower), and the SDS100 couldn't even approach that level of reception on distant LSM sites. Where the SDS100 got messed with by interference of various types, the G4 was unaffected. I couldn't monitor Glendale PD sitting in my house in Glendale with the SDS100 (G4 has full signal bars in the same place).
In the end, the SDS100 was a really neat device that didn't work out for me, but works out for the majority of folks in their situations apparently. If it works for you, you will have a real scanner, not a pager, and you will be happy. If it doesn't pick up simulcasts as you need it to, and those simulcasts are really important to you, you are likely to end up with a G4/5 eventually. They are purpose built for 700 MHz simulcast, and the only way you do better is with professional gear.
Oh, and there is a LOT of "fan-boyism" on RR as I'm sure you know, but I have no axe to grind or brand to shill for, this was my honest experience that mirrors the experience of several others in the valley, a few of whom I met in person to do head to head testing with. I love and use products from Uniden, GRE, Whistler, Unication, and other companies that make radio products. I use what works, and I share my experiences to assist others in my hobby in making informed decisions. Still, my experience is just one man's experience in one place, it is what it is.
And to save some the trouble of jumping in to say "but you didn't wait to install the firmware release that some say helped"....others on on RR in the valley who went on to use that and subsequent firmware releases report that the same issues continue on to this day, regardless of which firmware they install.
I would say buy the SDS100 from Amazon, where you can return for full value if needed, without even having to pay return shipping. Try it and if it works wonderful, you have a scanner instead of a pager. If it doesn't work, it costs you not a cent, and you move on to the G4/5.