Lots of reasons and ideas behind them all. I Love my Shaman, and I love my Beastlords. At lower levels, I think the Shaman would be a better fit, however, at higher end, I have had my Beast keep my groups alive when the Cleric wasn't working.
My 2cents...Beastlord, Reason: PARAGON. You are running a very high endurance using group, and there are very few things you can get that will actually help regen Endurance (as well as Mana and Hitpoints). the Beastlords Paragon and Focused Paragon AAs will (in my opinion) reduce your downtime by helping to keep your toons in the most desirable and hard to find substance called Endurance.
Also keep in mind that you Beastlord does have a very nice selection of caster abilities as well (Healing, DD, Dots, Debuffs), and the nice add of a tanking capable pet. Also remember that beasts do have minor forms of many of the Shaman spells pluss many abilities the Shaman does not.
Remember in the end it is what YOU want you group to do and/or be that makes a difference. Shaman are great backup healers/and buffers, and Beasts are well...beasts. Learn them both well and they will both do a lot for your groups.
Hope you figure out which will be best for you, and please keep us informed as to what your decision is (and maybe even why you decided that) as it my help others who are in a similar crossroads.