Having been on the field in India and China both I've seen the need for both. Some would say a balance, I'm not sure its a balance we need but an overflow of both. I think they go together, if there is intentionality in the training.
We have something people desperately need to hear, the Gospel... yet in many places we must EARN the right to open ears. We do not have the right to be heard, unless we earn it with love.
So do we do a construction trip or an evangelical trip? Yes. If you build a house you often have freedom to open the door and build the home too. It is difficult though. It takes extra work to do both service and evangelism.
That being said, one of the most powerful things I have ever seen is someone working sacrificially along side a non-believer and sharing the love of God while they scrub the sweat from their brow. Sweat and struggle opens the heart for words.
Let us look at the great commission. In the original language the idea that is given by Jesus is that while we are going we should make disciples. So often we either just go, or just try and make disciples. But if we went, worked, and each took part in the personal evangelism as we went, the world changes.
This is one of the weaknesses of short term trips, we have little time to work and share with people. This is why it is so important for short term trips to supplament long term local ministries. We can join hands and make a huge impact with our gifts. If we do it on our own, essentially we are bringing baby Christians into the line of fire and leaving them hanging when we go home... but really thats another topic
Suffice it to say, yes it must be evangelistic... but it can do that while being a service trip.
