Thin layers of near-surface nitrogen-vacancy (NV) defects in diamond substrates are the workhorse of NV-based wide-field magnetic microscopy, which has applications in physics, geology, and biology. Several methods exist to create such NV layers, which generally involve incorporating nitrogen atoms (N) and vacancies (V) into the diamond through growth and/or irradiation. While there have been detailed studies of individual methods, a direct side-by-side experimental comparison of the resulting magnetic sensitivities is still missing. Here we characterize, at room and cryogenic temperatures, approximate to 100-nm-thick NV layers fabricated via three different methods: (1) low-energy carbon irradiation of N-rich high-pressure high-temperature (HPHT) diamond, (2) carbon irradiation of d-doped chemical vapor deposition (CVD) diamond, (3) low-energy N+ or CN- implantation into N-free CVD diamond. Despite significant variability within each method, we find that the best HPHT samples yield similar magnetic sensitivities (within a factor 2 on average) to our delta-doped samples, of <2 mu THz-(1/2) for dc magnetic fields and <100 nT Hz(-1/2) for ac fields (for a 400 nm x400 nm pixel), while the N+ and CN- implanted samples exhibit an inferior sensitivity by a factor 2-5, at both room and low temperatures. We also examine the crystal lattice strain caused by the respective methods and discuss the implications this has for wide-field NV imaging. The pros and cons of each method, and potential future improvements, are discussed. This study highlights that low-energy irradiation of HPHT diamond, despite its relative simplicity and low cost, is a competitive method to create thin NV layers for wide-field magnetic imaging.