HP and Spd should be your focus. Bard/Sorc's strength is PVP, so there is no reason to ever use anything but an HP build. Use high defense equipment and trophies, fort your armor and get your Eidolons' defense stats as high as you can and you're pretty good to go.
Tigerius is the best Eidolon for a lure. Sigrun and Bahadur match up pretty well stat wise, but Sigrun is better. Aelius will be good for later in the game's content, Quelkulan is always a good one to have around for certain PVP situations (he'll also be your personal worst enemy as a Bard, but Bard/Sorc is well equipped to face Quelkulan). Bel-Chandra and Uzuriel are good team support for their buffs if nobody else is using them.