+1 Both!
Like others I use a blowoff tube the first 2 or so days of vigorous fermentation, then switch to an airlock.
I've learned this lesson well . . . I pitched yeast on one of my first batches one afternoon and put an airlock on. Next morning I got up, was groggy, watched some TV when I heard a weird sound from the kitchen. I went in and beer was spewing out of the top of the airlock. Now, I knew I needed to switch to a blowoff tube, so I grabbed a racking cane & hose. So I pulled out the airlock (I was groggy and not thinking clearly yet) and . . . .BEER GEYSER!!! The beer literally shot out thru the small hole in my drilled stopper all the way up to my 9 foot ceiling. (Luckily SWMBO was out of town that weekend.) I cupped my hand over the geyser so it deflected to the floor, got my 8 year old to get me my racking cane, shoved it in the stopper and put a pitcher under it. It was far and away the most vigorous and quickest fermentation I've ever had.
Anyway, there is good reason to use a blowoff tube first!