I hate to say it, but I find them both to be somewhat necessary in Indiana. Our transitional seasons are just SO transitional, with cool to cold mornings followed by pretty warm days. Try as I might, I just have had bad luck with leg warmers staying up. I like the knickers so much more.
I have two lengths of knickers. Some PIs that hit me just under my knee and some Shebeest that hit me mid-calf. I probably wear the latter more because I like the chamois and fit better. They see me through early to mid- fall and early to mid-spring. I also have some Sugoi tights that are a must for winter riding. I will say that I've had better luck with my lower end Sugois (I think it's the Helix) than the higher end Shebeest and Sugois that I have. Go figure. So, I don't think you have to break the bank to get a decent pair of tights.
But here's the thing: When it's really cold and/or windy, I wear both the Shebeest knickers and the tights. Having two layers over my knees really helps.
Keep in mind that a lot of tights don't have a chamois. I find the chamois- less ones to be more versatile, since you can layer them over something with a chamois.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher