I try to give a short answer.
There are historically 4 groups of SP regs:
- chromed brass (108, 109, 129, 156). The first is obsolete, the other 3 are my favourite ones. They can be all fitted with the balanced S-wing poppet of the G250, converting them in second stages that are almost the best you can buy, despite having been built 40+ years ago.
- central poppet: this generation started with the mythical Pilot (the regulator which is till now the easiest-breathing device ever built, but it was a nightmare to service and tune). That chrome beauty was soon replaced by plastic clones, such as Air1, and the long D series, culminating with the current D420, which is currently the top unit of the SP line.Expensive units well suited for very deep diving.
3) The G250 and all its variants, obtained on one side by declassing it to versions without effort knob and/or unbalanced, such as the G200, and on the other side improving the original design with many particulars, such as carbon fiber parts, titanium parts, gold-plated parts, and modern variants such as the G250HP and the G250V.
The G250 has been the most succesfull reg ever built, sold in large numbers and being manufactured for more than 30 years. Only recently the latter G250V was replaced by the G260, which is currently the SP reg tailored for usage by commercial and tech divers. This is what I usually recommend to buy to people asking advice on which reg they should purchase. My advice is to buy a MK17 or MK25 first stage and two identical G260 (I hate the idea of a lower performances secondary octo).
4) all the others, including the S series, the A series, the R series, etc. All these are models launched by SP for covering some niches of the market. For example the A700 is for nostalgics of chrome cans (but an upgraded 109 is still better, in my opinion), the S series are smaller and lighter for travel, the R series are octos with reduced performances, etc. I suggest to stay away from this 4th group, except in case you really need the specific features provided by them.