Now that we've narrowned down our search, the next step is to carefully and thoroughly go through the data sheets and application notes. A list of things to look for can be found here.

STMicroelectronics

See the STM discussion page.

Philips

See the Philips discussion page.

Atmel

See the Atmel discussion page.

Brief Summary of Differences

ADC

Atmel - 0 to 3.6V, 2us or 1.25us min conversion time (depends on ADC clock).

Philips - 0 to 3.6V, 2.44us min conversion time.

STM - 0 to 2.5V, 1ms conversion time.

USB endpoints

Note that endpoint 0 must be a control endpoint for packets from the host. I assume that this is true for these chips, although Atmel claims endpoint 0 can be control, bulk, or interrupt.

Atmel - 4 endpoints, with some flexibility for assigning types.

Num. endpoints	Type	Max Packet Size (bytes)
1	control	8
2	isoc/bulk/interrupt	64
1	control/bulk/interrupt	64

So, we basically have only two isocronous endpoints on the Atmel chip, and we're limited to 64 byte packets.

Philips - 16 endpoints, fixed transfer types.

Num. endpoints	Type	Packet Size (bytes)
1	control	8, 16, 32, or 64
5	interrupt	1 to 64
6	bulk	8, 16, 32, or 64
4	isoc	1 to 1023

With Philips, we have 4 isocronous endpoints, and a lot more endpoints to play around with. We're also not limited in isocronous packet size (1023 is the max packet size from the USB spec).

STM - 16 endpoints. There's no mention in the "electrical characteristics" data sheet or the reference manual of what type of endpoint these are, or whether they're fixed or set. Seems worrisome that they wouldn't include this information. There's also no USB application note, unlike Atmel and Philips.

Clock Generation

Atmel - 48MHz core and 48MHz USB clock, using a 12/6/4MHz (probably 4MHz) crystal oscillator.

Philips - 60MHz core and 48MHz USB, using a 12MHz crystal.

STM - 48MHz core and USB clock, with a 4MHz crystal.

PLL

Atmel - no mention of PLL settling time.

Philips - settling time is 100us.

STM - lock time is 600us max.