Both, dont put all of your water in one thing, if it gets contaminated then your screwed, with multiple storage options ur better set. Plus u might need to take out a couple gallons on a walk or whatever. Use multiple storage methods and sizes. And have a way to get more water and steralize it too
I started with Walmart gallon jugs, then moved up to the blue 7 gallons. Then I saved my money and got one, then two, then 3, 30 gallon. I went 30 because they fit my storage area.
The gallon and 2.5 gallon do not store well, they leak.
If you have the space, 55 gallon drums work well. Make sure you put enough chlorine in it to keep it from going bad, and get a barrel pump. That way you can pump out enough to fill a smaller container when you need to.
Water will be one of your most important resources, and you can never have enough.
Store it properly, with the right preservative - - plenty on this board to show you how to do that - - - learn to do a search before you post, and you will find that water storage has been discussed many many times.
I use 250 gallon tower drums in the front corners of my garage. I use 55 gallon drums along the walls. I recently saw a post on here where someone used something called a 2x4 basic that they got at Lowes, but it is sold elsewhere as well, to create a shelf capable of holding a second 55 gallon drum up over the bottom one. I'm working on that prep this weekend and will line 8 55 gallon drums along the wall with 8 more above - for a total of 16 drums on the wall (plus the 2 250 gallon tower drums in the corners by the garage door). And I can still get my cars into the garage!
Under the stairs, I use water bricks. Each one holds 3.5 gallons and they have a nice handle. I was able to stack 72 of those in a configuration under the stairs.
In other places, I use the 7.5 gallon watertainers. The only thing about these is that they don't stack very well - - I have only been able to stack 2 high otherwise they leak.
I also have an 8,000 gallon underground cistern tied to rainwater collection, and I have a minimum of 2 55 gallon drums tied in a series at each downspout.
I can also refill the cistern from my well, but it is about 400+ feet deep so it will be difficult sans electricity.
But like someone else already pointed out, use every size water container - - whatever works for the space you have. Small ones make it easy to grab and go, but the big ones are priceless when TSHTF.
Again - - do a search! There have been literally 1000's of posts already done about water storage and how to purify it, how to filter it, how to collect it - - you name it. The search feature will become your best friend
Do what works for you. The hard plastic clear bottles like sports drink or juice bottles work well. The milk jug type plastic jugs don't. It would be good to have large tanks for long term use and small jugs you can take with you if you have to leave.
Big 55 gal for rainwater collection planned use as of now is for sanitary flushing. 5 and 7 gallon containers for drinking and washing, as well as 2 liter bottles. Plus a whole lotta flats of bottled water.