Lump Sum. As other's have said, it makes the most sense if you remove emotion from the picture.
If you can't stomach that entirely, I'd do a hybrid. Put in a large lump sum now, then a few more large investments in the coming months. For example, put in 40% now, and then twice a month for the next 6 months, invest an additional 5%. You can structure it any way you want, but if that makes you more comfortable than putting it all in right now, it's not an awful decision because you are still getting much of the money working for you rather quickly, and you aren't spreading it out over such a long period that you are going to lose all that much of the growth and compounding.