I am new to saltwater reef systems but master at freshwater planted aquariums. My question is it worth buying the E260 Max AIO model and adding the sump upgrade or just go with the 260 Max without the rear sump? I want to go with soft corals and invertebrates but would like to progress to hard corals and wondering if it’s worth the money long term to have both?
I have had my E260 AIO (no sump upgrade) for about 7 months. I am happy with it, and I would buy it again. Prior to this I had an Oceanic 58 gal with a traditional under tank sump for 17 years. I have a mixed reef (softies, LPS, some SPS), with six fish (probably will add 2 more). I agree that you will be able to keep SPS in the E260.
I chose the E260 because I wanted to have everything fit inside the cabinet, especially the 20 gal RODI water reservoir that I have. For the small reservoir included with the E260 sump upgrade, you will probably need add RODI water every 5 to 7 days (also true for the Red Sea Reefer 250).
The disadvantages of the AIO:
1) As mentioned, you will likely have to live with the provided skimmer. Other people have managed to shoehorn other skimmers, but choices appear to be limited. The MSK 900 has kept up with my current bioload (nitrate 1-2 ppm, phosphate 0.8 ppm).
2) Red Sea won't allow you to substitute other lights with the package (I asked). On the other hand, the RS 90 LEDs appear to perfectly adequate to grow SPS
https://youtu.be/U3gImt5Zo5U
3) Without an under tank sump, you won't be able to explore potentially fun things like a refugium or filter roller

4) With an AIO, the noise from the reservoir (mostly the skimmer) is not muffled by a cabinet, as with a sump setup. I find the hum to be acceptable, others have not.
If you are going to go with an under tank sump, suggest the Red Sea Reefer series (or equivalent tanks from other manufacturers). From what I can tell from the manual, the E260 sump upgrade is a bit of a hybrid, where some the return water volume bypasses the sump because of the circulation pumps (others please correct me if I am wrong).
Also, another thing to consider is the kind of fish you wish to keep. For example, the display volume of the E260 (56 gal) will preclude you from keeping a full grown tang or even some dwarf angels.
Hope this helps.