With high mana cost champions like Malzahar, high mana cost, high base damage I think it's simply better for every laner save the ADC and Support to take Flask. You get out mana sustained by opponents who take Flask when you take Doran's Ring.
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Kouga4 votesThe Flask gives better sustain but the Ring is better if you're planning on an early fight/engage. A little bit of HP & AP can give you an edge in an early fight (prior to the first recall). If you're planning on trying to kill them its probably a better start. If you're just planning on poking/harassing, then it won't work very well against a flask (unless you can force them to use all their charges without draining yourself dry), and they'll have to either recall or risk an engagement without any actual stats.
disregardable1 votesDoes that mean 9/10 times you should take flask over dorans?
Kouga1 votesDepends on the match up.
If I were Le Blanc, I'd probably rather have the ring for AP to burst down and get an early kill.
Malzahar, on the other hand, isn't going to get an early kill, but rather just try to farm with E, so flask would be better sustain.
kJs1 votesYeah. Thanks. I've just been following his recommended early items. I'll try Flask starts now. +1
1 800 MID LANE2 votesNot to mention that a Doran's Ring gives mana sustain in it's own right, if only relatively little. Even so, that 3 extra mana for each last-hit gives a lot more than people realize. They just need to learn how to CS.
Kouga1 votesits 3 mana/5 seconds, and 4 mana per unit killed. Each minion wave has 6 minions (7 with cannon). 6x4= 24 mana if you last hit them all. 30 seconds between each wave: 18 mana/30 seconds. That makes it about 42 mana per minion wave if you get every CS.
The Flask gives you 60 mana per charge and only takes a few seconds to regen. 60x3= 180 mana for 3 full charges. So a flask will give you more mana than about ~4 minions wave with the Ring.
It also depends on how long you're in lane. Dorans would give you more mana long term if you are just farming in lane, but if you are recalling/teleporting back to lane, you'll have more sustain with the Flask's charges.
1 800 MID LANE1 votesNot to mention that a Doran's Ring gives mana sustain in it's own right, if only relatively little.
Kouga1 votesSorry I got distracted by math. I like putting numbers to it for comparison, wasn't trying to say you were wrong.
1 800 MID LANE1 votesApology not accepted. >=(
Penns1 votesLiterally the only situation dorans would be better than Flask is when you die without being able to use it. And since mid lane basically = poke until you can all in, how often does that happen? lol
ScriptBrand1 votesdoesn't malz have a mana refund mechanic?
HaIlMonitor1 votesWhen things die when his E is on them. You actually have to be pushing hard to get mana back though because it didnt last long enough to kill whole waves with just a few ranks in it.
ScriptBrand1 votesi mean...that's kinda how you play malz, contantly shove your opponent into turret...
HaIlMonitor1 votesYeah blow all your mana to get a little bit back!
Dediguise1 votesIf you are only using your e you can pretty much sustain infinitely with two dorans rings. Why not take 
instead?
Dorans ring is fantastic because it exceptionally cost effective and it gives you that early damage to push champs out of lane. and to survive burst/ ganks. Flask gives you more sustain, but zero stats. It's almost usless outside of lane phase and it sets your itemizaion back. Because you either back and buy dorans rings or your first major item. If you don't buy the dorans rings you are weaker in the trades when you return to lane after your first back.
Basically pick flask if you want to be safe pre 6 and can use the sustain to outlast and outfarm your opponents. If you don't take advantage of this, when you return to lane that flask will be far less cost efficient than the big damage item your opponent just got.
If you want your level 6 powerspike to dominate lane then go dorans ring first. Dorans ring actually helps you the entire game.
Again will allow you back without losing cs or to rush an early ap item to win trades. Maybe you could grab some mana pots as well if you really wanna be a dick.
1 800 MID LANE1 votesOn top of that, his E is expensive in it's own right with the relatively poor mana regen Malz has.
Sohleks1 votesThe dorans ring in actuality gives you more mana typically than flask if you farm well enough long enough. You certainly have a lot more hp sustain with flask though.
Usually ranged mages still prefer the ring for the superior mana regen. The flask is usually taken by melee champs to survive some matchups, or ranged champions with such weak early games they may just seek to farm. The thing is if you're a doran's ring mage with limited mana it WILL be hard to poke down the enemy flask user with limited mana pool (particularly ranged ones with good positioning...) as the flask potions start gives so much hp. You kinda just have to accept you'll likely just end up farming before first recall if you have limited mana pool and limited 100-0 potential.
The flask is often not as desirable as a d-ring mid game though... And in actuality if flask didn't exist massive potion buys are not that much less efficient. Can't nerf flask to the point it can't outperform regular potions...
Flask is actually secret op on ranged AD solo laners though as they can dish out a constant barrage of free harass. Trading hp is like a resource like this... 
What if flask had different values for ranged champions compared to melee ones. A straight nerf, or maybe a change to more mana emphasis. I'm personally fine with the way it is tho ;p
1 800 MID LANE1 votesThem taking flask generally means they're afraid of taking a lot of damage (i.e. Getting countered hard) or that they know they're bad at mana sustain. Taking a Doran's Ring should mean that you don't have those problems. Unless you just follow the recommended builds because you don't realize that there is more than 1 option to start a lane with.
AyRe CoNteMpT1 votesboth items have their distinct playstyle and use and there is no generalization of what is better.