Sure, Soulstealer is a matter of taste, I just think it's very easy to get stacks, either with assissts or kills, it doesn't matter. It just gives you in combination Death Cab over 200 AP more so, I just love it^^

Just take for your AD build for Seals manareg per lvl and then you don't need manamune, sure if you are not that good with Ezreal Manamune will help your early game but it is senceless anyways in end game.

Furthermore it is as Ezreal not your job to kill tanks. Never try to kill them in fights as first or second or third..... kill them with the rest of your team when you killed the damagedealers, you wont need to kill them alone, and for tanky dps champions like renekton, your Black Cleaver will be enough armorreduce.
To help it, just try armorpen runes in Marks and Quintessences. Attack damage are **** and senceless, you deal less dmg with them.

Ty for commenting but I don't aggree with soulstealer because if you own you just own even more with Meijas but it wont help you own it just will help you be better when you are good.
So that's just overrated, if you get as much kills so you get 20 stacks then you win the game with our without Meijas.

My AD build, Manamune is important, else you get oom, I don't play crit build because of the spells that stack AD(mystic shot and ulti) and Banshee's can be bought if you need it but you will not be able to kill tanks without last whisperer.

Hi,
I play Ezreal myself very much, at least over 200 games and I play both, AP and AD.
I like your AP build but i would just add a Mejai's Soulstealer cause you nearly get every game your 20 stacks full with him. Just buy it before your Sheen.
Now your AD build. I personally hate Manamune on him. Its just a waste of money. He has enough manareg. in mid/late game, better , no MUCH better would an Infinity Edge. You have very high crits and more attack damage. It's just more worth for your money. Last whisper is ok if enemies are stacking high armor but i would replace it for a Banshee's Veil cause you are always focused and this just saves you from e.g. an initiating stun like Ashes' ultimate or Amumu's/ Malphite's.
Further on you have enough attackspeed with Black Cleaver and Trinity Force, so get other boots, maybe Ionian or Swiftness.
I hope i could help you, adn as said with a Soulstealer added your AP build is very solid and usefull :P