Neither system is particularly interesting to me, and nothing compared to when DS launched. DS felt like something new, 3DS and Vita are more of the same which is okay-ish, but I'd prefer something different. Both systems have mostly uninteresting line-ups, with the exception of Paper Mario for 3DS, which is one of my favourite RPGs and looks amazing. Also, MK3DS with the potential for the gliders and new tracks looks promising.
In terms of sales, neither will be as successful as their predecessors.
Nintendo's strategy is flawed, adopting a luxury item type philosophy, similar to Sony really. Cost is a major factor. The system is too expensive and the games are far too expensive. Nintendo should be moving to a download retail space with games like Steel Diver and Pilotwings selling for around £10-15, with their retail equivalents in the £20-25 range. Online has been a mess thus far. Focusing on the 3D effect is essentially a graphical upgrade, with most titles simply selling on that premise alone, also there is the disadvantge of some people not liking it. Removing focus from stylus touchcreen gameplay is a mistake. Until Nintendo decide to sell 3DS a relatively cheap, around £130-£150 RRP, with cheap-ish sw, basically as a cheap fun handheld, they won't even get to GBA level success.
Sony's strategy is bizarre in terms of wanting to sell more or anything close to the same amount of systems as PSP. They've done little of significance to address PSP's biggest problem, selling original sw. Outside Japan (even then there are provisos), beyond a small audience of dedicated Sony fans, little suggests people want to play home console experiences on handhelds. Vita's graphics will not draw in a wider audience. The system has already lost a lot of PSP owners using their handhelds for media/pc type use to mobile phones and tablets. Development costs are also an issue. Outside Japan PSP is in an incredibly weak position, coming to market with what is essentially a 'better' version is doing little to help this.
Ultimately both are facing massive competition from mobile/tablet devices. Neither seems to address this. I also believe the days of the dedicated (or principally focused) gaming handheld are numbered.