Guily, you're spinning more opinion as fact misinformation again. There is no latency difference with PCI-e versus USB. Think about it. There's a lot of high-end studio equipment that interfaces with USB. EMU 0404 is a good example, heck I think it even had a PCI interface. If you want less latency, get rid of your useless CPU processing and use ASIO. That's what the professionals use for recording because they NEED 0 latency.
Think about it for a bit. If PCI-e was so much better than USB in terms of latency, gamer companies would make PCI-e mice with pretty lights to sell to customers as being the fastest mice ever. They don't because USB is plenty fast.
I use Reclock to synchronize audio and video playback on my computer for movies so if USB did have noticeable amounts of latency I would see it because the audio and video is synced when it leaves the CPU. Yet, the graphics go over PCI-e and the sound travels through 10-15ft of USB cable into a DAC, through a tube, into an amplifier and out the speakers in the same time. Everything looks and sounds perfectly in sync. How is that possible with your logic?
*EDIT*
Found this on Wikipedia:
PCIe sends all control messages, including interrupts, over the same links used for data. The serial protocol can never be blocked, so latency is still comparable to conventional PCI, which has dedicated interrupt lines.
I will say that PCI-e is better for studio work where you have a lot of channels coming and going and USB 2 would be overloaded. But, for 99% of computer use (including gaming), it doesn't matter.
You made one good point about that DAC being too big/heavy for your laptop for portable use. If you want a serious one, check out Fiio, they have portable DAC's that can run on batteries or off of USB power.