Generally, the fastest connection wins. However, before Windows 7 networking was a mess and performance was awful all around. Try to get more than 20mb/s from XP good luck with that.
The standard TCP congestion avoidance algorithm employs an additive
increase and multiplicative decrease (AIMD) scheme. When there is no
packet loss detected (by means of three duplicate-ACKs or
retransmission timeout), the congestion window (cwnd) is increased by
one Maximum Segment Size (MSS) every RTT. Otherwise, if a packet loss
is detected, the TCP sender decreases cwnd by half. In a high-speed
and long delay network, it requires a very large window, e.g.
thousands of packets, to fully utilize the link capacity. Therefore,
it will take the standard TCP many RTTs to recover the sending rate
upon a single loss event. Moreover, it is well-known now that the
average TCP congestion window is inversely proportional to the square
root of the packet loss rate.
A faster connection has more successful packets and therefore its cwnd/mss is increased and it gets even more of the total connection.