Been to both countries last year. Security was perfect in both and I loved both of them. Maybe Nicraagua due to your time period and it being a smaller country. Enjoy

My brief ideas:
Nicaragua - Cheaper, easier to get around as smaller, safer than Colombia but not to say its safe, very well developed tourism in major sites: Granada, Leon, Ometepe etc Easy to go hiking in Matagalpa, Jinotega, Esteli area with easy transport from capital.
Colombia - Beautiful scenery (all types...), more friendly, better infrastructure but much bigger, better music, parties, more culture, much more dynamic cities, bigger mountains, better fruit juices!
Go to one this time, and the other next time.
