New planets take ages to build up to something useful anyway.
Indeed, but DG strikes me as a long-term game, so colonising is definitely useful. How about a balance between them? 100k on arcs, 200k on fleet? Colonise 6 planets with 2 arcs each (108k), then spend the rest on a fleet? That way you get a semi-useful planet by Christmas which will hopefully help to bolster your now growing empire
