Indoleacetic vs indolebutyric acid
 

I wonder why the common rooting hormone products seem to use indolebutyric acid instead of indoleacetic acid.

In at least some cases IAA is preferable:
Plant Cell, Tissue and Organ Culture, Volume 49, Number 1 - SpringerLink

http://nopr.niscair.res.in/bitstream...%20448-452.pdf

Pulling the cobwebs off of my memory of plant tissue culture experience from over 20 years ago... IBA is more chemically stable than IAA. A commercial product would have a longer shelf life with IBA and be less sensitive to storage conditions, and also less affected by pH or other environmental conditions at time of use, more fool-proof as a product for use by the general public or in field conditions as opposed to laboratory use.

I can´t reach the first article but in the second it seams that they actually compare the effect of IBA and IAA. Though it does not seam to be the main focus of the study...

At uni IAA is also always kept in a freezer and on ice during use

The first study compares the effects of IAA, IBA, and NAA. Here is the abstract for you:

"We have examined in vitro rooting of apple ‘Jork 9‘ shoots exposed for three weeks to each of the three auxins commonly used for ex vitro rooting: indole-3-acetic acid (IAA), indole-3-butyric acid (IBA) and α-naphthaleneacetic acid (NAA). During the initial five days of the rooting treatment, the cultures were incubated in darkness. In this period, the root initials are formed. Then, the cultures were moved to the light. NAA resulted in a low (ca. 8 roots), and IAA or IBA in a high (ca. 15 roots) maximal root number. The maximal root number was reached at a wide range of IAA concentrations (10-100 μM) but at only one concentration of IBA (10 μM) or NAA (3 μM). With NAA and IBA, growth of roots and shoots was much more inhibited than with IAA. For these reasons, IAA is the preferable auxin for in vitro rooting of apple ‘Jork 9’ shoots."

The other thing is that this study found IAA to be best in in vitro rooting. They do stress in the discussion that ex vitro and in vitro are two very different rooting procedures, and explain why IBA is a better choice for ex vitro:
"For ex vitro rooting, auxin is applied by a short dip in
auxin solution or rooting powder. Thus, auxin is only
taken up during a brief, initial period. This is comparable to the 24h application shown in Figure 4. In this
case, IBA is preferable, because IAA is rapidly oxidized by the plant tissues (Epstein and Ludwig-Muller, ¨
1993) and very high concentrations have to be applied
(Figure 4). Furthermore, IAA is more susceptible to
microorganisms than IBA (Hartmann et al., 1990)."
David- I think that this paragraph gives a pretty good answer to your question.

I will add an observation: I suspect that not only does the environment play a role (in vitro versus in vivo), but which hormones affect which plants is likely to be very specific.

I have used K-L-N (5000 ppm NAA + 1000 ppm IBA, + 200 ppm B1) for years, and I have done controlled experiments that show it is effective at stimulating growth. The "standard" dosage I've heard recommended is about 2 drops per gallon, which results in 0.21 ppm active ingredients.

After some experimenting, I recently have switched to using KelpMax. It is much lower in hormone content (11 ppm total auxins of 7 different indole-3- compounds - none IBA or IAA - and 0.031 ppm total of 12 different cytokinins), and used at 1 tablespoon per gallon, the result is 0.04 ppm of active ingredients - 20% of that in K-L-N - yet it is far more effective in eliciting a response from the plant.

Thank you everyone for all of this good information. I also found this 50 year old book and ordered a used copy from AbeBooks/Amazon:
Auxins and Plant Growth - Aldo Carl Leopold - Google Books