The new round cables are the best choice. They combine the electrical advantages of the flat 80 wire cables with the neatness and enhanced airflow mentioned. 18" is the maximum cable length allowed by the IDE standard. Longer lengths are non-standard and may result in data corruption or reduced speed.
80 wire cables have 40 pins, just like the 40 wire cables. Otherwise the cables wouldn't fit in the same receptacles. The extra 40 wires on the 80 wire cables are ground wires between the conductive wires, and are used to reduce RFI (radio frequency interference) between adjacent conductive wires. This configuration improves data integrity compared to 40 wire cables and enables higher speed capability of up to 133 MB/sec. The standard recommends 80 wire cables for speeds of 33 MB/sec up, although a 40 wire cable may be used up to that speed.
UDMA Mode 2 (33 MB/sec) and UDMA Mode4 (66 MB/sec) devices may be attached to the same 80 wire or round cable with no concerns. Modern IDE controllers have individual device timing so that either UDMA device will run at its full rated speed.
I wouldn't pay extra for a "True 133" cable. A decent quality standard round cable should work fine up to 133 MB/sec.. Be wary of additional enticements which may be marketing ploys. For instance, some cable suppliers may list separate 80 wire categories for 100 MB/sec (more expensive) and 66 MB/sec. The specification is the same for both, so don't fall for paying a higher price unnecessarily.
You can get the round (3 connector) 18" Vaster cables rated for 33/66/100/133 MB/sec starting at $3.00 at CableClub.com.