Two very different topics! on the surface both are very broad and not clear. Please dig more in the literature to find detailed info, make a table of key information you find and check the lab facilities and your capabilities. Electrospining is a very populated field, for doing a research in this field a very thorough lit review required to find a gap.

about me I would nominate the nanofluids because you can doing good progress especially if you find the gap of the topic that you mentioned (solar collector with nanofluids) this will be such a challenge. regards