I would get the griddle...two burner size, leaving one burner open for pot or skillet.
2nd accessory would be the deluxe BBQ box...cast iron grate, heat diffuser...can also be used as a shallow oven for pies, biscuits, corn bread, etc. and pizza.
Word of caution...use only low heat settings with the griddle. (and/or never loan it out)....My first griddle came back from a weekend loan warped so bad it had to be scrapped. The new griddles come pre seasoned and the reinforcement underneath seems a bit heavier than my old one.
I have both a pro 60 and pro 90 with folding legs, ~30 years old and still working great. The only 'issue' is size and weight...suited for cargo trailer types vs the average van dweller.