There is a good side to keeping the druid's hitpoints... you can turn into tiny creatures and still have your normal hp, but massive bonuses to attack and ac.
It's worth considering, also to cut down on bookeeping, that you can have your summoned creatures not 'actually' attack, but flank AND 'aid another'. They just have to hit an ac of 10, and whoever they are aiding either gets +2 ac against the enemy, or +2 attack against the enemy. For a fighter with power attack that translates to a +4 damage.
For Summon Nature's Ally I, the best monsters for this purpose are the Eagle with a +5 attack, and the Octopus, with a +5 attack. Basically, this means, 80% of the time these creatures will provide a +2 bonus... so... 1.6 per. Alternatively, you can summon 1d4+1 with SNAIII and get basically +5.6 on average. The ac of the target creature has no effect on this bonus.
Oh, keep in mind there will probably be a +2 flanking bonus as well.
As for the animal companion, you can use it to gain flight at either 1st or 4th level, depending on how the GM interprets Natural Bond and the higher level animal companions. A dire bat is a Large creature with 40 flight, good maneuverability. It gains abilities as if your druid level were your druid level -3. Some GMs interpret this and natural bond to let you gain access to higher level animal companions three levels in advance. Regardless, you either gain flight 4 levels or 1 level before anyone else. You can have fun with this and ranged weapons or ranged attack spells, like produce flame. This is also a good way to cast SNA spells. You could argue that the Dire Bat gives you soft cover from enemies beneath you. Casting while mounted requires a laughable DC 10 concentration check. 15 if your mount is 'galloping'. Have you ever seen a dire bat gallop?
If you decide not to use the animal as a mount, I have been informed by a powergaming friend, that the tiger is the ultimate animal companion. You could take a level of beastmaster, and take natural bond, and it would be a *truly formidable* animal companion. I played in a game with this friend and his tiger, and the tiger tended to be more effective in combat than the druid.
Also, keep in mind... your animal companion is EXPENDABLE. The only thing that happens when it dies is you have to spend 24 hours to get a new one. That's it.