As an owner of both of these machines, I can confirm the following qualities:
eMac Points:
When maxxed out, this eMac will offer speeds comparable to the low-end 1.6ghz iMac G5. It has fantastic built-in speakers with great bass and sound quality, and thanks to the overbuilt cooling system, the GPU clockspeed and VRAM can be overclocked up to
33.33% with the help of ATIcellerator II. And if you can track down its tilt and swivel stand, that will multiply the style and function factor by
10x, giving you all of the beauty and function of the iMac G4.
However, it can be a pain to clean out dust and access the HDD / ODD (so make sure to give it an SSD when you do). You will also often need to readjust the CRT display after moving the machine around. The single fan is also relatively loud. It is rather hot-running. It is very heavy, and there are no handles, so you will need to
MAKE SURE it is packaged
VERY well when it is shipped to you, otherwise the case will be prone to cracking. Thanks to improper packing, my first eMac arrived with a cracked bezel (which then needed to be replaced). And due to the proprietary implementation of its CRT display, no version of Linux 2014 and newer will run properly. The same applies for OpenBSD, so your OS choices will be totally exclusive to Tiger and Leopard.
Otherwise, they're great all-in-ones, make for a fantastic sound system, pack a formidable punch, and are all-around excellent starter Macs. Just keep in mind that they're happiest on your desk, and just about nowhere else.
Power Mac G3 Points:
As this is a Power Mac, its room for expansion and potential blows away the eMac. It is much quieter than the eMac. It is insanely easy to overclock (no soldering, just jumpers). The CPU can be upgraded to a higher clockspeed G3 or G4, and the graphics card can also be upgraded, all ending up to something more akin to a Power Mac G4. PSU aside, it exhausts very little heat, therefore they are relatively cool-running. Even with 512mb RAM / the stock Rage 128, this 400mhz machine is very responsive on Tiger when given an SSD. On the previously mentioned configuration, it will browse the Web reasonably painlessly with Camino 2.1.2 if JavaScript is off. It has the same speaker as the iMac G3, so it also has very good sound quality (the eMac will still trump it in this area, however). Unlike the eMac, this machine will also run any version of Linux or OpenBSD without any issue, opening up far superior security, far broader browser choice, and greater sheer all-around ability.
However, this Mac has a sulky personality. It has a
very sulky personality. It will often play favorites with RAM modules and optical disc drives. If a module happens to be installed that it doesn't like, it will once in a while throw a fit and kernel panic. It plays better with single-sided modules than dual ones, though, so make sure you buy single-sided, unbuffered,
Mac-specific, 168-pin PC100 / PC133 RAM modules. If it has a ODD it does not like, it will refuse to boot from certain discs. Being that this is a Middle Earth ROM machine (NWR, but has remaining elements of OWR), it will not boot from USB or FireWire, only disc or HD. There is no Target Disk Mode, though it supports reading / writing to other TDM machines mounted as external drives. There is no boot picker (opt key at chime), but only the Open Firmware
multi-boot utility, and even then, it has to approve of its drive configuration if you don't want it to hang. It only has two USB 1.1 ports, so make sure to get a 4-port USB 2.0 PCI card. It also only has ATA/33, so also get either an ATA/100 PCI card or SATA PCI card.
Otherwise, they're great machines, with great speakers, with great expansion and potential. They will give the eMac a run for its money if properly upgraded, are more energy-efficient, and are all around excellent DIY Macs. Once they have a RAM, ODD, HDD, etc. configuration they like, they won't give any problems and will be extremely pleasant to use.
IMNC's Power Mac G3 is a great example of one of these guys taken to their full potential.
I (really) hope I helped.