Leves are technically faster. You get the same exp as you would with grinding, plus the leve reward exp.

I usually do a mixture. I'll do a leve and blow all my GP on it, then grind inbetween leves waiting for my GP to come back. This also gives me some stuff to sell. A lot of people like to save their leves for DoH instead, but if you're like me where your allowances will always be maxed at 100, use leves whenever on whatever.