It really depends on your budget.
Edgar Markov was the face commander in the Commander 2017 preconstructed deck, but because he's only ever had a limited print run with that product (and because Eminence is broken), he alone is over $100, and I doubt you're going to be able to find a sealed copy of his deck (Vampiric Bloodlust), let alone for a reasonable price.
The other thing to
consider is what kind of deck you'd like to play and your preferred play style. Edgar is pretty straightforward. It's vampire tribal, and I don't think there's too much variance in how people typically build decks around him: cast vampires, make vampires, make vampires big, swing. The +1/+1 counters can get kind of annoying to keep track of because of the number of tokens you're making, but that's a minor issue. If you sit down at a table with Edgar as your commander, understand that you probably will be correctly assessed as a significant threat before you even start playing. If it wasn't for his Eminence ability, even if he cost 1 or 2 less to cast, he wouldn't be nearly as oppressive.
Extus/Blood Avatar probably does give you a bit more flexibility with deck design, but it's also more difficult to build well - although there are several three or four card combos that go infinite and win on the spot (though I typically don't like playing infinite combos). Ultimately, it is an aristocrats style deck: sack a bunch of your creatures to drain your opponents of life, recur them or make more, rinse, and repeat. The Magecraft ability on Extus turns out to be more of a useful incidental thing than it is something you can reliably build the whole deck around (although it would be interesting to see it done that way). In reality, you're probably actually not going to be casting Extus all that much but rather making a bunch of tokens to sacrifice to Blood Avatar and keep recasting that. It's not as threatening as Edgar would be and will likely be cheaper to build. However, keep in mind that, because Blood Avatar hurts all of your opponents equally, it can incentivize the table to collectively deal with you.
Personally, if it were me, I'd go with Extus, if only for the cost. I can't fathom spending over $100 on a single card, especially for my first ever deck. The Eminence mechanic is quite imbalanced and not exactly fun either in my opinion. I wouldn't worry too much about the deck not being "strong" just off Arena testing. While it can be a bit more clunky than Edgar would, it can certainly hold its own in most pods. That being said, if the people with whom you play are OK with proxies (most people are in most cases, especially when playtesting a new deck), proxy both and play both. See which one you like more.
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