Which SpiderIV do you have? Do you get a sound(s) you like out of the speaker? If so, then miking the amp is the best bet. If its the 15 or 30 models, then the osund is probably not that full from the speakers, so the DI route (amp's DI/headphone output to Scarlett 1/4" input) may be the best route. Note that the headphone output is stereo (even with single-speaker Spider models), so if you are using any stereo FX, you will want to split the headphone output to left and right channels to separate inputs on the interface.