1/4" vs 6mm

I live in the US, and I’m looking for some cheap 1/4" bits that I can experiment with so I don’t have to constantly live in fear of pushing things a little too far. I’d like to experiment with different sizes of bits, especially with a beefier router and the (almost finished) upgrade to a Promo.
The problem is that the cheapest 2-flute carbide bit/end mill I’ve found seem to be about $15+. Looking at the usual suspects for cheap import stuff, all of the promising ones seem to be either 6mm or claim to be 1/4" but are really just 6mm. I’ve ordered a 1/8" collet from Elaire, and that said I could still modify the order, so…
Would it be worth it to add a 6mm collet to open up access to the cheap 6mm bits, or should I just stick with the more expensive 1/4" bits?
Or, even better: is there a good source for inexpensive 1/4" bits?

Hqmaster bits from Amazon and even bits that Ryan sells are affordable and pretty good quality. I would stick with standard 1/8 and 1/4 bits as they are much easier to find.

Hqmaster is one of the places that seems to have only 6mm bits and no 1/4". Honestly, they mostly seem to have 1/8".
And Ryan seems to also have nothing bigger than 1/8".

I got a 6mm collet from elaire while I was ordering for my 611. I’ve got some 6mm endmills and I’ve used them a little, but really I rely on the 1/8 for most of the work. It’s a pretty good workhorse size, really. On my smaller machine, I’m usually working in metal so I don’t want to cut away more than I have to, on my bigger machine rigidity is a little less than I’d like so I probably couldn’t push it harder if I wanted to.
I did smash the 660 1/4 collet around the 6mm mill a few times and it seemed to hold up OK, but I don’t know how long that will be true (haven’t used the 660 in a while either).
IMO, if you really REALLY want to use 6mm to save some bucks, spend on the collet to hold them correctly. If you just want to see how they fit into your workflow, spend the $$ on one endmill and see if you even like using it.

I mean, I have used 1/4" bits, but they’re expensive enough to make me super nervous about breaking them, so I don’t really think that I’m using them as efficient as I could.

I think the collet was only 20 or 30 bucks. Sounds like you should just do it, or you’ll always wonder, lol. FWIW, the two 6mm mills I got were something like $2 each, HSS.

The irony is that there are TONS of suppliers (and even manufacturers!) of these bits nearby. I’m contacting a few to see if they’ll let me pick em up. Saving on shipping brings them down to $8-10 each. That’s not too bad.

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If the point is just to experiment bigger size with cheapo bits, then why don’t you just go for the 6mm ones?
Once you finish experimenting with these, just switch to the 1/4 stuff and you’ll be good to go.

That way you purchase cheap 6mm ones and a more expensive, higher quality 1/4 for whenever you’re ready, so you won’t break the expensive stuff. The size will be a tiny bit different but it won’t affect your feed and speed by much, you’ll be in the ballpark already.