Which is better,100 half duplex or 10M full duplex?

Depends on what you're using it for. Technically 100 half duplex would be faster than 10 full duplex but it doesn't account for the collisions that will occur on the segment. A collision occurs when 2 devices try to transmit data at the same time. If you're just checking e-mail or doing normal surfing it's not a big deal but if you're trying to stream HD video the collisions can become very annoying as it will distort the picture. Also collisions can take up to half of the bandwidth of the connection so during heavy use periods you might only be getting 50 from your 100M line. Though 50 is still better than the 10 full duplex, at least you won't have collisions.