The choice depends on available instrumentation and expertise, and some what less on the peptide structure. If your peptide is big enough, or if it includes some feature to limit structure, then you may observe characteristic alpha and/or beta absorption using CD. A short peptide, with only one beta strand or part of a helix, may not fold stably in the absence of a partner.

I recommend you minimize Cl- ion, as it exhibits high background absorption. I suggest you also predict secondary structure based on primary structure, using any of various computer tools.