Im tooling up for my bt30 setup and have a few questions.

First of all, er vs sk: Seems that most go with er collet style but i question why. After doing some research, it seems that sk collets are better in just about every way. The taper is half the angle of er which results in lower runout, better grip, higher rigidity. Only downside i see is that the clamping range of each collet is a half mm instead of a full mm like er. I dont really see that as a significant problem since i prefer to go with exact size collets anyway for best contact. My core tooling stays mostly the same so im not really spending more by having exact collet sizes for each tool. I also looked at price of sk vs er holders and they are very close. Is there anything else i should consider for sk vs er?

Collet size: whether i go with sk or er, there are multiple sizes to choose from. Is it best to hold a tool in a collet type closet to its size? For example if im running a 3/8 tool, does it matter whether i put it in an er16, er20, er25, er32? Obvious factors would be cost and clearance issues from a large nut if i needed to reach down into tight places, but other than that, is there other considerations? Will a larger than needed collet style generally have more runout?

Gauge length of tool holder: I would assume minimal is always best but it seems the most commonly sold sizes are much longer than they need to be and sometimes not even available shorter. For example in bt30 er20, 70l seems to be the most popular length and its very rare to find shorter. I know they can be made shorter, ive found er20 50l, but extremely rare and very expensive. Why is this? I have found er25 in 45l to be somewhat common and reasonably priced so that ties into my question of smaller tools in bigger collets. So considering a 3/8 tool, will it be better off in an er20 70l, or an er25 45l?

When i bought my spindle, i got a killer deal on 10 er20 70l holders to go with it, but before long i would like to have 30 or 40 total, so i need to figure out what i want. For how cheap the 10 i got were, its not too late to just keep those as my less commonly used holders for odd tooling and standardize around sk for all the new ones if it makes sense to go that route.