Which two feats do you think it is worthwhile to lose?
Assuming you still want to use a spiked chain, Multiattack will not have much use, because of the difference of reach between your weapon and your bite attack. And even if you tend to go closer for using your natural weapon, that is just a +3 attack bonus to one weak bite attack.
Iron Will may not be that important if you take more levels in DD, as it gives you higher will save.
Personally, I am not a big fun of spiked chain and improved trip but it does depend on each DM's habit of how many and what kind of opponents he tend to use. I will write about this below.
+3 LA makes me think twice about that.
As already pointed out, you don't need to worry about it.
It's worthwhile to invest a small amount at least, since charging will happen.
Forgive me, but I just don't see multiple AoO coming up in the games that I play, while Tripping has been shown to work fairly consistantly.
Well, that kind of experiences differ largely between play groups.
In my 10 or so years of 3.Xe game experience, charging has never been a significant maneuver. It often happens that not a single PC charges even once during, say, a 8 hour gaming session.
It depends on how strict a play group use the charging rule by RAW, how wide or complicated the battlefields are, how close the encounter distance is, etc.
Same can be said for AoO. If a DM tend to use small number of monsters per encounter, multiple AoO per round will be a waste. But most published mods tend to use encounters against massed-monsters several times in an adventure.
And If a DM tend to use a lot of medium-sized two-legged opponents, and if many of them are melee warriors, then tripping would be useful.
But if your DM likes to use larger, multiple-leged, or totally untrippable creatures (say, ooze) or spellcasters or something non-melee, tripping is almost useless.
So, usefulness of those feats and tactics will depend largely on your DM's style.
