Going from ER to normoxic will add only the ability to use helium. That means gas planning, deco theory regarding helium use, isobaric counter-diffusion theory and practice. Thats all classroom stuff.
In water, you will add (possibly) a suit bottle if you are diving drysuit and your ascent rates will be less forgiving than, say, an air dive.
If you are planning to go full trimix anyway, the argument can be made that the intermediate course won't really be of much benefit. However, since you also seem to be unsure of your current experience level prior to going to the next step, I would say that doing the normoxic course will give you one huge benefit, that of being able to reduce narcosis on the practice dives you will be doing between now and full trimix.
Of course, training costs and travel etc etc do play a role, but if you can, get helium first and then look toward going deeper with it. As to the original question, Advanced trimix will remove the O2 percentage limitation on your card so if you didn't do the normoxic course, you would still be able to get any trimix / helitrox mix you wanted.
In practice, once the blender sees "trimix" on the card they stop asking questions. I have gone in with my normoxic card and gotten fills for a buddy of some very low O2 mix without an eyebrow being raised.
