@JMahncke In what situation? These are 2 very different cards that function, usually in different kinds of decks. Even though they have a similar control mechanic I don't think they are really easily comparable. What deck are you playing that would have to make a choice between these 2 cards? It would have to be 4 color minimum which isn't unheard of.
But I think they generally fit into different builds to begin with.
I guess he means in general for Vintage, considering the fact that many BUG decks play Leovold instead of Notion but Notion has it's own deck based on him. Rather than a given context I think Notion Thief has a better spectrum of use and versability (in Vintage) but that's just my opinion so far.
As said, it depends on the deck. Having flash, I prefer notion in decks with lots of instants and other flashy creatures. Works nicely with draw-7, but also with fayden.
Leovold works at sorcery speed and demands 3 colors, but 1 less mana. It looks great with deathrite shaman, easily playable in T2 after it. Unlikely notion, it does not give advantage directly, but drawing a card each time something yours is targetted is nice. Better in the long term, while notion can be awesome stealing gushes, ancestral, jace&fayden activations...
Both die to common removal, though leovold can block more creatures and survives snuff out.
I personally prefer notion because of the flash clause and because cavern on humans is safer than cavern on advisors. But probably Leovold is better overall.
I've been pretty good about smelling a Notion Thief blowout coming and planning accordingly, but the few times it happened it was pretty devastating.
Holding up four mana is a pain in the rear and that's part of the reason that it's probably not seen more than it once was.
I have zero experience with or against Leovold because MAGIC ONLINE IS A QUALITY PROGRAM. It's too bad too because I have ideas I'd like to try with the guy.
Neither card is very well positioned in modern Vintage.
Thief has its place in the metagame in my opinion. It's not a tier1 card, but definitely very playable. I have grown to respect the card.
However I am not sold on Leovold. I just dont see it having much of an impact. I mean Notion Thief is a small player in a big pond, Im not trying to convince anyone how great it is. It has it's place and role in the format, whereas Leovold has lot to prove still and I have my doubts.
Leovold's second ability is the big reason to play him. Its obviously great against discard effects, and generates value off of removal. A deck like Grixis therapy would be much happier to see a Notion Thief than a Leovold (not that they'd really be happy to see either).
The Shops MU is where there are major notable differences between Notion Thief and Leovold though. You draw a card for every counter removed from Triskelion that's pretty insane, he has 3 toughness letting him block a lot of the smaller creatures, and he costs 1 less CMC making him easier to squeeze down under spheres. Where Notion thief is pretty much useless in that MU, Leovald seems a decent piece of a control deck.