As in where the sm-500 fits in between the juniors & seniors, & what the difference between senior & sr ii is.
I could but it’s complicated. You will need to wait till when I’m not last minute gift shopping for christmas 
Haha of course Seana, not in a hurry!
Ok so here we go.The SM-500 is functionally the equivelent of the Senior. The difference being the SM-500 has management features, it records things for up to 500 patrons, treatment intensities, number of insertions etc . The SX-500 is the upgraded version of the SM-500, with perhaps a few more modalities thrown it, but still a 13.56 MHz epilator. The Senior II came out a tthe same approximate time as the SX-500 and has all the same current features. Essentially, the SX-500 and senior II are just new format of the same machine.
Then we have the 3G, the platinum, the Platinum pure, and the Xcell, in order by date of manufacture. The 3G is an updated version of the senior II, still a 13.56 mhz epilator. I havent used one, so I dont know the advantages it may have.
Starting with the platinum Dectro started using the 27 mhz frequency . All of the platinum, platinum pure, and xcell have a “pro” version that includes more advanced modalities ( such as Synchro) ,The pro version often includes the management features previously employed in the SM-500 and SX-500 models, but with one key difference in the electronics.The SM-500 and the SX-500 have a battery on board the memory card which has to be changed periodically to maintain the firmware ont he machine. On other models like the Senior, Senior II and 3G, thie memoryt is maintained by a battery located on the motherboard in a chip called the RTC ( Real Time Clock) chip. The disadvantage of the rtc chip , is that once that battery runs flat ( approximately evcery 10-12 years) either the motherboard in the epilator must be replaed, or its got to serviced by Decrto to solder on a new RTC chip… The SM-500 and SX-500 models have a slight advantage over the senior and senior II models with regards to the battery, since youcan change the battery witht he machine oin and the card inserted ( so it doesnt lose power) and without soldering onto the motherboard, but this functionality comes at a cost. If you do forget to change the button cell int he meory card and it goes flat, it loses all of it’s volatile memory and is not functional after that. .Worse, Dectro no longer sells the SRAM memory cards ( which look a bit like a PCMCIA card) and if yours has gone flat, your SM-500 or SX-500 are quite hefty paperweights at that point. I have an SM-500 in my basement that is in exactly this condition! IT used to be the backup epilator in my clinic, until I discovered I had ignored it for too long.
If I was looking to practise every day and use the machine in a production environment, then I would not consider the senior, senior II, SM-500 or SX-500. The battery issue is a genuin problem that will stop you dead in your tracks.If you do plan on using an SM-500 or SX-500 I would set up a maintenance schedule every 6 months to a year to replace that battery. Because once it goes flat, you epilator is a doorstop unless the motherboard is replaced. This goes doubly so for the senior and senior II which is even more difficult to replace because the component is soldered on board.
Seana, I assume the cleo models have the same battery issue—but do the platinums & xcell?
to be honest I dont think so, but I’m not an engineer involved with Dectro. I will say I havent heard of this issue on the platinum or xcell.