I spent the large majority of my competitive riding as a speed athlete on the velodrome. I spent a lot of time in the gym and on the velodrome. I couldn't climb a hill fast to save my life. I was very good in flat fast crits., but even a small climb on the course put me under stress. Size will help your flat out speed, but will negatively effect your ability to climb. There is nothing wrong with doing both. I tend to ride with guys who wait at the tops of hills or slow down to regroup. I need to get back in the gym. I won't lift like I was when I was sprinting, but there are a lot of health benefits to weights.