Even if the s/s version does a decent job of approximating the tone, the AC15 will have the response, the touch-sensitivity, that has been the enduring quality of tube amps for decades ... you can 'play' a tube amp as much as you play the guitar.
If you have the choice between these two items, there really is no choice. Use the item that is the original, not the one that is an imitation of the original.