That is an invalid comparison. Dragon Aspect is a BUFF, an active effect with a 24 hour reset time, while Slow Time is more like a temporary attack/defense boost, with a very small cool down.
You don't use JUST Dragon Aspect. You use Dragon Aspect (5 second cool down) THEN a shout. Which buffs Slow time by a magnitude of 1.0 I believe.
Dragon aspect -20% CD, Amulet of Thalos -20% CD + Stop time with 3 words. With this combination you can literally stop time, burn down 3 - 4 enemies with your chosen method. Wait 5 - 10 seconds, then Stop time AGAIN.
Hands down, Slow Time is the best shout. Especially so when coupled with Dragon Aspect. It is the only shout I have found that lets you boost every offensive style, yet still manages to offer a small enough cool down that you can use it effectively multiple times in a fight.
You can slow time and;
Archery: fire 10 - 15 arrows, killing any number of people. I like to slow time when i have archers on the walls around me and snipe them all, an arrow for each, before they get off a shot. Cant zoom though, during the slow time so that is a con to note.
Melee : Two handed, or one handed, you can drop 3 - 4 harder enemies, and 6 + low hp enemies before its duration finishes
Assassin : I have managed to use 30x backstab damage on up to 7 guys before it runs out. I do have shadow warrior perk though. Add the nightingale power that turns you invisible when hidden and you actually feel like some bad ass ninja.
Caster : you can get off 3x firestorms, or lightning storm can tic for about 30 extra tics during slow times duration. Fire off 12 ice spears, which then fly at a slow speed. This is HUGE fun at long range. You basically create a train of ice spears that then all zip to the target at once when time restores itself. Use wall spells, which is hilarious, you slow time then paint the entire room with the wall spell of your choice. The caster options are endless.
Dodging: Slow time and run around fireballs, duck and weave around shouts, even sidestep arrows, as you charge your opponent to close distance.
Fleeing : You can basically exit combat very very fast as well, if you so choose to.
Show me another shout that has THIS many options? That results in you avoiding nearly ALL damage yet also lets you significantly boost your own damage, all with a low enough cool down that you can use it multiple times in a battle. This is all on Master skill lvl of course.
You approach four enemies, shout, run through shredding them to stand on the other side. Their now dead bodies still falling as you sheath your weapon, then snap. Times back, and they all fall to the ground.