~더라 is stating your opinion (and doesn't necessarily mean you disagree with the other person). 여자친구 보니까 아주 예쁘더라 (could be use to simply express how you thought your friend's GF is beautiful. Your friend may not have shared his opinion with you) on the other hand, 여자친구 보니까 예쁘던데? (implies your friend may have mentioned that his GF is not very pretty. You are opposing to his opinion in retrospect--meaning, you probably saw her some time in the past, and now you are telling your friend about your opinion from the past experience) *~더라 can also be used in a similar sense (disagreeing with someone else's opinion by stating your opinion), however the intonation while be different. I am sorry I can't describe it here :-) Hope this makes sense.

I just give you sentence not grammar. 너 여자 친구를 보니까 이쁘던데요. -> 여자친구 이쁘시던데요. (This is much better smooth to listener.) (Your sentence had respect speaker and not same location his girl friend) 1. Your girl friend is beautiful when I see As you knew many expression exist or could be made, only different is situation like above my tip. I hope so you could make sense my intention.

In fact, I hardly see the difference between 더라 and 던데. Anyway, the dictionary's definition is as follows. 1) 더라 Used when a speaker conveys a fact that s/he experienced and came to know newly to others. 2) 던데 Used when a speaker talks about a certain fact or event of the past with a feeling of surprise or admiration and wants to hear the listener’s response.