When I have a choice I always choose the long-tail cast on. The only trick is making the "tail" long enough. Perhaps see how many inches it takes for you to long tail cast on twenty stitches and then multiply it by 7. (Just to give you a tail to work with at the end.)
Long tail will work for most things. The knit cast on you are doing will work too. I don't really think one or another will be easier to work with once you get it done. A knitted type cast on will prevent you having to start over if you miscalulate the length of the tail.
For a knitted cast on, I usually prefer the cable cast on where, after you have two stitches, you knit between the stitches to cast on. I like the way it looks better. But it is up to personal preference. A tip: If you do a cable CO insert the RH needle between the stitches before you pull on the stitch just added to snug it up.
Tip I just discovered while trying it for you: If you do a regular knitted CO where you knit into the stitches... after you pull the new stitch up and put it on the needle, you don't even need to take it out and replace it for the next stitch. Just let the RH needle go back behind the LH needle after you have used it to put the stitch on the LH needle and you are ready to knit the next stitch. Wow, I never knew you could do that. :-) Makes it super easy. Maybe everyone knew this but me. LOL
One thing to consider is that garter fabric tends to work wider than some other st patts. Also, you want the CO/BO edges of the blanket to be fairly stretchy. Knit and cable COs look nice and are sturdy but don't offer much stretch. I'd recommend long tail for its elasticity.