ur better off doing it together. That way you only slow the plant down once rather than twice. As it gets bigger and veges faster (they really start growing fast when they get some maturity), then topping doesnt seem to slow them down much at all. But ur fine t transplant whenever you want to, just be aware growth may slow for a week as it builds more root stock.
Also look into LST and Super cropping. It has the benefits of topping without the stress. Geta a grow or two under you belt and do some experiments to see what suits you, your space and your style.