Cyclocross (small) crank and road cassette?

or

Road (big) crank and mtn cassette?

What are the disadvantages/advantages of both?

For cyclocross racing a 39t inner ring and 46t or 48t outer ring combined with either a 12-25 or 12-27 cassette is the most common (and effective) setup. There is usually very little climbing on most cyclocross courses and low gears aren't needed. The tight 1 or 2 tooth jumps on the road cassette is important, since accellerations out of corners, etc is critical in cross racing. Many folks use a single chainring setup, 42t seems pretty popular but it depends a lot on the course and the strength of the rider.
A 53t road chainring is too big of a gear for most folks. When it's combined with a 12-32 or 34 cassette it is still pretty awkward, as it will really only be useful when combined with the easy half of the cassette. Consequently, it is easy to end up in the 39 the whole race, which will also suck since the jumps between cogs are too great on a mtb cassette and you won't need gears that low in races either. In short, the 53-39 and mtb cassette gives you a lot of unusable gear combinations, and the steps of the cassette cogs aren't close enough to be efficient.
Throwing on a 46t chainring and adjusting your front derailleur height is about all it takes to get ready for the race season, gearing wise. Sometimes you may have to remove a link from the chain as well. That's all pretty easy though, compared to getting a longer chain, a long cage rear derailleur and a mtb cassette.

For cyclocross racing a 39t inner ring and 46t or 48t outer ring combined with either a 12-25 or 12-27 cassette is the most common (and effective) setup. There is usually very little climbing on most cyclocross courses and low gears aren't needed. The tight 1 or 2 tooth jumps on the road cassette is important, since accellerations out of corners, etc is critical in cross racing. Many folks use a single chainring setup, 42t seems pretty popular but it depends a lot on the course and the strength of the rider.
A 53t road chainring is too big of a gear for most folks. When it's combined with a 12-32 or 34 cassette it is still pretty awkward, as it will really only be useful when combined with the easy half of the cassette. Consequently, it is easy to end up in the 39 the whole race, which will also suck since the jumps between cogs are too great on a mtb cassette and you won't need gears that low in races either. In short, the 53-39 and mtb cassette gives you a lot of unusable gear combinations, and the steps of the cassette cogs aren't close enough to be efficient.
Throwing on a 46t chainring and adjusting your front derailleur height is about all it takes to get ready for the race season, gearing wise. Sometimes you may have to remove a link from the chain as well. That's all pretty easy though, compared to getting a longer chain, a long cage rear derailleur and a mtb cassette.

oh word. haha. that made the decision easy. :-P