The 1st idea is a lot more novel than the other. I haven't seen a time travel webcomic yet and it combines two uncompetitive genres; historical and sci fi which means you'll be serving a niche that isn't being catered to. As for the 2nd idea, I've seen a few stories similar to it. The time period is the only thing that makes it different.
Buuuuut ...I think that the 1st idea would do better on Webtoons than on tapas and the 2nd one might do better on tapas.
Personally, I'd definitely read the 1st one. If your art is anything like your avatar (assuming you made it coz most artists make their own avatars) it would surely have great potential