While a flask isn't absolutely necessary, using one makes pouring loose powder into the powder measure much easier to control the powder flow down to the last few grains.
I prefer the Traditions brass flask because for me, the plunger valve is easier to use than CVA's flask with the lever/spring design. The Traditions plunger spring is thinner and I'd rather not need to stretch out my thumb to depress the lever, so I just prefer the button plunger instead.
The plastic pour spout for the can is more likely to pour too much powder and there's no precise way to control the flow. Even though it would work, it's designed to refill flasks from the bulk container.
It's usually better to have any flask than none at all.
The flask's long thin spout tube also helps to precisely direct the powder into the powder measure, especially if the powder measure has a narrow opening. And once the volume of the flask spout is determined, it can also be used to measure out powder charges more quickly in the field, and different volume spouts can be added.
http://www.traditionsfirearms.com/eshop/10Expand.asp?ProductCode=A1201
This TC flask has the same plunger design as the Traditions but it doesn't have a threaded, interchangable spout:
http://www.cabelas.com/cabelas/en/t.../search-box.jsp.form23&_dyncharset=ISO-8859-1
Every brand of powder has advantages and disadvantages. Most folks end up trying different ones over time. But if using loose 777, it's recommended to also use the Winchester 777 primers along with it because they burn less hot which reduces the possibilty of a crud ring forming. Using the hotter standard 209 muzzle loading or shotshell primers increases the chance of a crud ring forming with 777 powder, and so does loading with 777 pellets.
Loading with 777 powder requires using 15% less volume to equal other powders.
TC also makes a cleaner/solvent that's specifically designed for 777 that's named T-17 Bore Solvent.
http://www.cabelas.com/cabelas/en/t.../search-box.jsp.form23&_dyncharset=ISO-8859-1
http://www.olibuy.com/tcpic7488.html