I think both the 3DS and PSV will be less successful in terms of hardware sales than their predecessors.
The DS, because it's just so insanely huge.
The PSP, because while it "only" sold 70 million units worldwide, I think a lot of early sales were because it was the promise of something new - great (for the time) 3d graphics on a portable.
But ultimately the 3DS will do much better on the strength of Nintendo titles. Pokemon obviously, but I would not be surprised to see them pull something out of their hat that catches fire.
I think the PSV will do better than the PSP in terms of software, because it seems to have fixed many of the complaints about the PSP (2 sticks, UMDs, ghosting on the screen). But at the same time, I think people are just fickle and were looking for excuses in many cases, at least in the West, where people prefer to play games on their comfy couches and on big screens.
And the same time, the software line up doesn't seem to be that compelling unless you happen to be a PS3 fan, and if you are a PS3 fan, why wouldn't you rather play most of those non-exclusive titles on a PS3.
And there is a disturbing lack of 3rd party support (yes, I've seen the list, and no, I'm not impressed, especially as only a few 3rd party titles have actually shown screens). Hopefully this will pick up at the TGS.
And what is going to happen when the PS4 comes out, and it won't be quick and easy to port things to the PSP?
It will be the same thing all over again. The PSP used to get get PS2 downports. When the PS2 died, it got the occasional Wii port, but it was pretty much crickets from Western publishers, unless it was money hatted (like Assassin's Creed).
And then on the low end of the market, which only seems to be growing and growing, you have Sony and Nintendo competing a little, but at a price point far higher than on iPhone.
Very few people will pay $2-4 more for the same title on a handheld just because it happens to have buttons. Some, yes, those that really like buttons and those that don't have an iDevice.
And the ever present danger (like has happened with DS and PSP) of full fledged titles that cost $40 on the PSV/3DS selling for $10 and under on iOS will undermine both handhelds. It will happen. People will notice. Maybe it won't bother the hard core, those that pre-order games because they want it now. But I think those sort of gamers are getting less and less common.
It will hurt the PSV more, because they are more reliant on third parties.
