Even though its not really relevant to anything, I think its kind of an interesting question. 

Both are fairly successful systems which have so-far failed to get to a critical mass. 360 has sold a fraction of what PSP has, but at this point is the only one of the two which has a chance to still reach that critical mass and become a mass market system. However, PSP got off the blocks faster and has a stronger worldwide showing.

360. There is far more developer support with major games than the PSP.

However, it'll be close, the PSP should get somewhere near 40m to even 45m before a succuessor is announced. The 360 should surpass this number by 5-10m units.

I think the PSP will stall around 30m. Maybe it reaches 40, but probably 35.

 360 I think will stall around 25M. Not too bad for selling in 2 of 3 markets (trying in all 3). Maybe they make 30M but I cant see much more than that.

I think after this Xmas we should have a good idea.

I don't think the 360 can do better than the PSP. Not with low sales in Japan. That will keep some of the killer games off of their system, or prevent them from getting certain exclusives.

I think the same, the PSP could reach the 40m mark, but for the 360 is going to be between 17m-19m this year and I doubt that they can better than that so I think that the 360 will end like 32m-35m at most. it can surpass if it becomes the next gen system winner(something not too obvious right now).

It looks like the 360 will not sell well in Japan at all, having only shifted less than 500k units in 16 months whereas the PSP has sold nearly 6m in 27 months.  Sales in the US are very good for the 360 and only lag 2m behind PSP, but in Europe they only have half the sales so far.

The 360 is currently on 10m units compared to the PSP's 21m+.

With the recent PSP pricedrop and a pricedrop due over the next 6 months in Japan and Europe, I can see sales of the PSP climbing a little over the course.

I can't see the 360 getting near 20m units worldwide for sometime, maybe another 18 months plus anyway, but it all depends on what happens with the PS3.

I will work out what I think will happen later. 

End of 2007 - 360 - 15m, PSP - 28m  

End of 2008 - 360 - 18m, PSP - 33m 

End of 2009 - 360 - 20m, PSP - 36m

End of 2010 - 360 - 21m, PSP - 38m

@ devygee

Your assumption looks pretty sad for both PSP and X360... I don't think we will see such spectacular decrease in sale during years 2008 - 2010. I think sale of DS is so good that Nintendo will not provide new pocket for much longer period. If there will not be a new DS... there will not be a new PSP too (I mean new because lite version is very likely). I think both systems will keep strong till at least 2012. I expect PSP to sell more than 50 millions before an end.

It's just my opinion.  I can see it going this way.  Sony will need to reduce the cost of the PSP to around $130 (£99  or less in the UK) to get a dramatic upsurge of sales.  I can see this happening, but not for another 12 months anyway...but if they do reduce the cost all round and we see an influx of new games due to high development costs on the 360 and PS3...then the figures could look different to ones I posted earlier...and could increase the sales by 50-100%.

Although I can't see the 360 selling more than 25m units in it's lifetime. 

360. The 360 sales will continue to increase year over year for the next few years, especially with the upcoming price cuts. The PSP sales are going to start to fade. It will take a few years, given the lead the PSP has over the 360 in wordwide sales but the 360 has a brighter future IMO.  While the 360 is all but dead in Japan, it has the largest userbase in Europe and North America currently. The 360 support is growing, PSP support seems to be fading.

Sounds possible. The PSP will end at 10 million units sold in Japan I think but the N64 sold better in the US than the PSP did till today and the N64 reached the 20 million mark.

My thoughts for the PSP:

Japan 10 million

America  15- 20 million

Europe 8 -10 million

I expect it to sell between 35-40 million units in it´s lifetime.

For the 360: The US numbers are actually higher than the numbers of the first Box were and the XBox sold 16 million in the americas alone. Also the XBox was launched one year after the PS2 so the lifecicle of the 360 should be one year longer.

My thoughts:

Japan: 1 million

America: 20 - 25 million

Europe: 8 - 10 million

I´ll just throw in the number 32.93 million consoles - that´s what the N64 sold and I expect the 360 to reach similar numbers.