The difference between a Hummingbird II and Dragonfly D5 are:
The Hummingbird has a 370 main motor extra tail supports and fin.
The D5 has a standard 280 main motor. It has all in one electronics (ESC, receiver and giro one one board)
All the parts that fit on one will fit the other.
In deciding which one to choose, you need to look at the whole package. Often the Dragonfly is available with lots of extras at a lower price than the Hummingbird. Don't forget the same basic design comes under various names some cheaper than others.
Personally I'd go for the cheapest option with the best included extras and then upgrade the motors etc as they wear out.