Ok, so the general Consensus is that SM is stronger than SNA. But the question is, just how much more powerful? For this comparison, I am going to suggest comparing the First-Worlder Archetyped Summoner vs a base summoner to give the SNA the best chance. Additionally, we shall assume both summoners have Superior Summoning and Augment Summoning. The SNA caster will have the XLight feats well, to give SNA the strongest chance.
So with these parameters, how do they actually compare?
As per unofficial word-of-god, First-world summoner can have her eidolon out AND any number of SLA summons out.
A better comparison is First-World summoner vs Master summoner
Then compare if the animal eidolon + SLA SNA is equal to SM SLA.
IMHO: I give it to the master summoner.
The only reason i picked First-Worlder was for their Expanded Summon list (which includes things like Unicorns). All I care about is purely the Summoning ability.
As I recall, there are three factors that push Summon Monster of Summon Nature's Ally.
1) Special Abilities: The Summon Monster list has lots of outsiders with unique special abilities and spellcasting/spell-like abilities. The SNA list, by contrast, is mostly limited to animals who can't really do much other than smack things with melee attacks. The only options that have options beyond basic melee are elementals, which are on the SM anyway. Don't get me wrong, putting animals in melee with the bad guys is very useful, but it's only one option. Summon Monster can do that just fine, plus more.
2) Templates: There are a pretty good number of animals on the Summon Monster list, and they all have a major advantage over their counterparts on the SNA list; animals summoned with SM gain the Celestial or Fiendish Template. Those templates will add DR, energy resistance, SR, and a 1/day smite, which is a very nice set of bonuses.
3) Communication/intelligence: Unless you have some ability to communicate with animals, it's going to be rather difficult to give instructions to summoned animals, and even with it animal intelligence is going to be a limiting factor on what they can understand. With Summon Monster, a few points in linguistics will let you talk to most of your summons, many of which have the intelligence to understand much more complex instructions.
Second, Chengar is correct for the most part. On his point #3 however, it is worth noting that the templates do not give the creatures a language any longer so you're still having difficulties communicating with them.
Gah, forgot that changed in the 3.5->Pathfinder changeover.
That derp aside, Angels, Devils, and Demons do make a pretty big chunk of the SM list (and are often the best options for their spell level).
For the 5th level spell and up, the major difference is between outsiders for Summon Monster and giants for Summon Natural Ally. The giants tend to be tougher but less versatile than their outsider counterparts.
One reason I was including the Xlight feats ie because those are feats specifically for SNA, and msot any SNA based summoner would grab them (in addition to Aug/Sup sum that ALL summoner's (trope,, not class) get). Is there any other feat that helps augment SM?