8GB sounds good to me. I probably won`t download many eshop/VC games, so 8GB might be enough for me and if i want more i can always buy an SD Card.

if you are comparing USB2 to SATA3 HDD on a laptop(5400 or 7200 makes almost no difference in this case due to HDDs reaching performance cap, where SSDs have not, even with SATA6), you are looking at 2-4x faster on the laptop setup.

Nintendo likely goes this route, first and foremost, for durability reasons. I believe flash memory is more durable than putting a harddrive in there. Hey, maybe I am wrong, but I believe that is the case. It is likely cheaper also.

you forgot to mention the difference in seek time, optical drives have terrible seek times in general so if multiple assests are being loaded, HDD would be faster anyways. If you are crazy enough to use SSD on USB2, which I don't recommend, it'd be even faster. SD Cards also have faster seek times as well, SDHC Class 10 only does about 10MB+/- per second though, not the most ideal for data streaming heh, but very good seek and burst buffer rates.

The key things to remember when we are talking about flash drives(and SD cards) are:

1.) Capacity (low vs HDD)

2.) Capacity Cost per Dollar (higher vs HDD)

3.) Actual Data Transfer Speeds (general lower than HDD, usually slow cause companies go el cheapo)

There are more advantages ATM to go full fledged good USB HDD than flash drives when we are talking about a console like Wii U if you want a better game play experience.

Good, consoles should never have tried to standardize hard drives in the first place. Less moving parts = better! Also, SD cards are much more convenient anyways.

Now go back to cartridges, so that consoles can last forever and have all of their traditional advantages against PC gaming back again.

As gumby_trucker excellently explained it really wouldn't be. It would still be faster than playing from disc on 360/PS3. 

Because the point is that playing a game from an external hard drive isn't going to hinder performance anyhow. So the transfer rate is fine. Slow would imply that it is slower than playing from a disc and would be an issue. It isn't. You could install a game to an external hard drive and it would run faster than off disc etc. 

I don't see any issue at all with the Wii U having USB 2.0. It isn't going to cause problems.

Also unless I'm mistaken those naughty people who took apart their 360s found that the 360 Hard Drive had a transfer rate of about 32MB/s. Which would mean a USb 2.0 hard drive would be between disc performance and internal hard drive performance. 

My aim was only to address the potential costs of having an external HDD, and none of the benefits, which is why I entirely skipped mentioning seek time and other parameters where optical media is at a disadvantage to begin with.

One issue regarding this which I did not address is that of fragmentation. I don't know how Sony or MS handle this with external hard-drives but they must have some precautions in place, otherwise there is always the risk that eventually seek times will become too high. I'm sure Nintendo is taking such precautions as well.