If I hadn't been to either one yet, I would go to Siena (one 'n') first because I like history and Siena being much larger has much grander things to see which appeal to me. The Duomo, Baptistery of San Giovanni, Museo dell'Opera del Duomo, Piazza del Campo (main square), Palazzo Pubblico (town hall), with its massively tall tower, Torre del Mangia. Santa Maria della Scala (museum/hospital/pilgrims hostel), Cateriniana Basilica of San Domenico, the city walls still standing and gates that really I never hear of anyone talking about. Siena is 2nd only to Florence is being one of the two major city-states that shaped Tuscany and what Tuscany is today so it's a powerhouse in the history of the region and in the big top 5 to see.
Lucca -the town of 100 churches, on the other hand, which I've also been to many times is wonderful but being smaller and less historically it would be on my see after Siena list as it's on the lower tier historically compared to Siena which looms large on the scene of history of Italy. Lucca's walls are the opposite of Siena in that they are the star attraction of this city. It's called the Passeggiata delle Mura Urbane, and has been turned into a green belt around the city to walk or rent a bike and loop around the 2 1/2 miles for a wonderful travel experience. All of Lucca's sights are smaller and much less grand than Siena and for many, that's the appeal. The Duomo of San Martino is less grand but still historic and you can go to the top of its bell tower which you can't do in Siena. The Torre delle Ore - Tower of the Hours you can climb also and from there you can see the other famous tower the Guinigi tower and it's oak trees growing on top. You can go to the Piazza dell’Anfiteatro and notice it's oval shape since there used to be a Roman amphitheater there. One you remove both citie's duomos from the list, there are more interesting churches to see in Lucca than in Siena if that's your thing. We never go to Lucca without a stop at the Cioccolateria Caniparoli, our favorite sweet shop in Lucca.
So I can only tell you what I would do based on what I like, what you like you have not said, other than a slight reference to aversion to crowds which can be present at both cities and will be proportional because of their size. Both of them are high on the day-tripper itinerary so both will transform after 5pm when most of the tourists disappear. It's Tuscany so there is no wrong choice, only a choice based on what's important to you which really is a bit of a mystery.