Both are good bases, with great restaurants and cultural offerings. Lucca is definitely bigger, then is connected directly with train to move around. SG is smaller but also has a lot, but to get around you first need to take bus to Poggibonsi and from there catch another bus or train... so it really depends on the day trips you were planning. How many days? SG is more central, so if you were planning on visiting Siena, Chianti or the Val d'Orcia, the time to get there will be less so that is definitely something to consider.
But if you were considering on focusing on the northwestern area of Tuscany, Lucca would also be a good base.