1.5 is fine when it's new and tight. Get a little wear on it and it's not worth a hoot. 3 is more forgiving, especially if you'll cut any small grains with your header.
Another problem with the 1 1_2 in. cut is that in no-till beans into corn, the old corn stalks don't fit thru and push over bean plants. I would not be afraid of buying a used 1 1_2 in. cut 1020, CIH has packages on sale at times that include just about everything you need to change it over to the 3 in. cut.
I don't know if you are intrested but I have for sale a 30' 1020 header with 3" cut.If you need a trailer to haul it I would sell that as well. I want $10,000.00 for the header and $12,000.00 for the header and trailer. It has been stored inside for the last 3 years because I have not used it. It is a 1996 model with the oil bath wobble box. I am in SE Saskatchewan. If you want more info call 306-783-2001 I could deliver it for a fee. Forgot to mention this would be in Canadian $$ don't know what the exchange rate is these days but it must be at least 10% on U.S. $$
I like the cut on the 1 1_2" better than the 3", but if you have any corn stalk or corn root residue at all, you'll be much happier with a 3" cut. The stalks and roots tend to push on 1 1_2" leaving bean stalks standing. At least that's been my experience.
$10,000 Canadian = $ 8,482.48 US $12,000 Canadian = $10.178.98 US Rate of exchange as of 01_26_07 from quote.com as per my Mac OS X currency calculator widget.