Fair Trade

The ratio of 0.1479 to 0.3617 is 0.40890240530826651921481891069947 to 1. So if you're trading them at silver content, you will be more and more accurate with the following sequence of ratios:

40%: 2 5 22 225 247 ... 3617
90%: 1 2 09 092 101 ... 1479

The mathematics of this is "continued fraction expansion", which for the rational number 1479 / 3617 is:
0; 2, 2, 4, 10, 1, 2, 1, 3.

Otherwise, I'd put some small premium on the 90% because they're probably easier to purify. On the other hand, the 40% also have valuable copper and nickel I guess.

I think this might be a close trade although the face value is more of a factor when silver is much lower in value.
If silver is very low, I would want to hang onto a few more 40% half dollars because 50 cents is still 50 cents should silver become close to face value, but when it is high as it is now, I think it makes little difference.

I think one roll of 90 % bullion silver is equal to 2 rolls of 40% silver and 5 additional 40% silver halves.

40% or 40 x 45 = 1800 or $22.50 face
90% or 90 x 20 = 1800 or $10.00 face

( I would think the same would hold true in face value for common date circulated Washington silver quarters and common circulated Roosevelt dimes for that matter.
2 rolls of dimes or one roll of quarters is worth about 45 40% half dollars as well. )

Hows that sound?

There are two reasons to prefer 90% to 40% halves.

One is that, with 40%, you need more than twice as much storage space to keep the same amount of silver.

The other is that if you choose to ship a bulk lot somewhere for refining, you pay much more in shipping charges.

Those scenarios might or might not apply to your situation. Even so, there aren't many reasons to prefer 40%. I don't think a swap based only on silver value is fair for the one giving up 90% and receiving 40%. I would discount the value of the 40%ers a little bit to get a little more actual silver value.

Just my opinion,