First, Welcome.
Second - if you are going to make other things that just cabinets, I would use that in my decision. I have just about every sander Festool makes except for the 150/5 orbital and the ETS 125 (I do have the EC 125). I think that this is a path most of us have been down before and you have some good advice so far.
I have changed my thinking on sanders over the years, but I will give you some of my recent thoughts. If you can swing two sanders, then I think that an RO150 is where I would start and then add either the RO90 or DTS 400 - to me, getting into corners is important for so many projects. I just picked up an LS130 and so far I have really liked it for doing shapes and edge/profiles. Many do not like the sander and it is a more specialized tool, but worth considering if that is what type of work you do. Having these three sanders allows you the flexibility to tackle virtually any sanding project.
One consideration is size - many experienced users say bigger is better because of the faster stock removal with more surface area of the larger disk. While this is true, it might not be the best choice for the projects you have in mind. If just cabinets, then the 150 size is a great option. However, I build many sculpted pieces and for that work, the RO125, RO90 and RAS 115 are more important because the size of the pad lets me get into the curved portions easier. If doing overhead work, then consider a smaller lightweight sander.
If I could only have one sander, then I actually would give a nod to the RO 125 -- because the size would allow me to sand smaller pieces easier and still allow me to work on bigger pieces. But, again, if I could get two then I think that the RO 150 and RO 90 would ultimately be my choice to cover the most bases with two machines. I get great finish results with these sanders and honestly for most projects I find the slightly larger 5mm orbit on the RO150 does not make a difference to my eye.
Other sanders to consider are the RS2E for large finishing needs when you want a flat surface - this is great for table tops and other larger projects. I do like the 150/3 and the 125 EC as a finish sander, but use my Rotex sanders much, much more for the type of work I do. I think that the DTS 400 is a gem of a sander and an orbital is useful for so many things. I actually really like the RTS400 too - cannot get as close as the DTS 400 for corner work, but you can add an interface pad for contoured sanding and you can switch the sandpaper around to extend its life - so something else to consider. The RAS is an aggressive sander with finesse and perfect for heavy sculpting work and taking paint off, scribing, etc...