My best advice would be two-fold:
First, learn to make the cowboy chords (first position chords) with not only the 1,2,3 fingers, but also with the 2,3,4 fingers FROM THE BEGINNING. This sort of flexibility makes the switch to barre chords much easier when the fingers are already programmed to form the chord correctly.....eliminates a the learning curve associated with switching to the 2,3,4 fingers.
Second, base the determination on whether to use barre chords on the chords immediately preceeding and following the chord in question. At times during the same song I will play the same chord both in open form and in barre form, it really depends on how easy it is to get to the barre chord from the preceeding chord as well as how easy it is to get from the barre chord to the following chord. A splendid example of this is the G chord....many of my songs seem to use this (G and D are my favorite keys).....I'll play with an open G and a barred G (using the E formation with fingers 2,3,4 barred at the 3rd fret) in the same song....
Barre chords are an intermediate issue....if you're a beginning player, don't feel bad if you have trouble with barre chords (particularly if you've made the 1st position chords with only the 1,2,3 fingers, as do most beginners).
YerDugliness, Esq./Post No Bills
Guitar Playin' FOOL, retired & attempting to age disgracefully!!
Washburns: WD32SW, D61SW, and C124SW
Other fine acoustic guitars by Breedlove (custom shop Revival Series 000), Darren Hippner (#506), Takamini (2005 LTD), Epiphone (Masterbilt AJ500RC), and Yamaha (G231-II)