It's all about personal preference. Will you feel the difference? YES. Will it be a positive or negative difference? That depends on you and what you prefer. I personally like 170mm, 175 just seem to long of a stroke to me and 165 feel to short. As Ed said, he prefers 175 for the longer stroke and more lever arm you get out of the extra length. It's all about you. Just switch them out and see how you like it, that is the only way you will find out if you like it or not. You can get a crank puller for around $10 on amazon and just do it yourself.

However, from my experience, if the nuts that hold your crank on is loosening it would probably be because of your BB and not the crank. Also, riding it loose will cause the crank to become whollored out meaning you might wanna consider replacing your BB and crank and not just the crank.