So for a given number of dollars the smaller drive will have better performance.
First, this is a red herring, since the price you pay for an SSD in a given size class won't buy you any significantly larger drive. So, a 60GB dog of an SSD for $60 is still far faster than the zero IOPS you get from a $60 120GB SSD. What you really need to compare is the cost per GB, because then you can compare things like the performance of a pair of 60GB drives in RAID-0 vs. a single 120GB.
That said, the primary factor in SSD speed is the number of controller channels that can be connected to the flash chips. For an example, see pretty much any review (like this one). Because of this, smaller drives always have lower performance. Even crossing manufacturers/lines can only rarely make this untrue, as a doubling of size doubles the channels, so the flash on a smaller drive would have to be more than twice as fast to make up the difference. And although you are correct that SLC is faster than MLC, it's not twice as fast.
So, if you can find a larger drive that costs less in total dollars than a smaller drive (and it is possible...there are a few 120GB drives that cost less than 60GB drives), in every case you will get astoundingly more for your money, as you get more storage and more channels used on the controller, which gives you more performance.