Using CR is not a good tool. CR measures the challenge the creature provides as a foe, but does not take into account the benefit the creature might provide as an ally nor the reduced abilities resulting from summoning (no teleportation, no summoning, etc ...). As a gross example: A creature that can cast heal at will, but has no attacks, would probably have a fairly low CR, but would be something way too powerful to be granted by any SM spell.
Using ECL is another option, but it fails to take into account the limited duration alliance and reduced abilities of the summoned creature. ECL is based upon the value a creature will provide as a long term ally. If the creature is only present in the short term, those long term benefits may be valued inappropriately. As an example, if the creature can cast a spell with a long duration, that duration ends when the summon spell ends, so the value of that ability is overemphasized.
Both, however, provide a good guideline. If the ECL of the monster is greater than the level of the lowest level PC that could cast the spell, be cautious of adding it to a summon monster list. If the CR exceeds the CRs of other monsters on the list, be cautious of adding it to the list as well.