Bleach vs. Hi-lift

I personally really don't have a preference.  It all depends on how much the hair has to lift and what you are wanting it to do.  If someone wants an all over blonde and has natural hair color of around 6-7, High Lift is great.  Any color less than 5, I usually use bleach and toner.  With highlights, it depends on the dimensions you want.  Sorry to be so vague.

I like both, but if you want a reliable blonde, especially if you want ashy or a toned blonde, you should go with bleach and 20 vol. Actually, it has been found that bleach and 20 vol. is gentler than a highlift (double 40 vol.). Now, if you don't use double 40 with highlift, it will be more gentle, but you won't get all the lift. What's good about high lifts is that they make the hair shine -- I used to use them on my Level 1 hair and they come up to a beautiful copper brown (always using ash shade). Sometimes highlifts do not even work on Level 6 and higher; every head is different.

I would like to color my platinum blonde bangs one shade darker, and lift the rest of my hair (which I estimate to be about a level 7) to a pale blonde.

Should I buy a darker toner to make my bangs darker? Right now the only toners i have are Clairol 323 (extra-light platinum base blue-violet) and Fanci-tone 49/10BV (ultra white minx).  Also, the only developer I have is 40 vol - should I buy 30 and/or 20 vol?

To darken your platinum, I would go with, like, a Level 9 neutral blonde (deposit-only) colour. This will likely take you to a Level 8 and then fade up. Not sure if Clairol Toner (other than Towhead) can take you one level darker?