Determining the best resolution and aspect ratio will depend on the quality and composition of your footage. The most common approach is to upscale the SD footage and maintain the 4:3 aspect ratio, then fill the letterbox area with background copies of the left and right portions of the same video with colorization effects applied to be less dominant. This should allow the SD footage to be OK at 1080p.
If you prefer to use full frame instead of the approach described above, you will need to assess the composition of each shot. Can the 4:3 footage be cropped to 16:9? Anamorphic 4:3 SD should be OK when resized and cropped to 720p 16:9 with square pixels. It might be acceptable when scaled to 1080p 16:9, but the source quality would need to be very high. Alternatively, can the the 16:9 video be cropped to 4:3? If none of these approaches is satisfactory, perhaps you can split the difference and go with 3:2 (720×480, 1080×720, or maybe 1080×1620).