^That doesn't make the decision any easier.

How much playing time do they have each roughly?

based on personal experience Tactics is longer with greater variety of things to do during and after you finish the game, the class system is very indepth and encourages you to use them as enough useage of one earns you new ones where as with Arc, there really isn't much encouragement to mix and match your team, you'll just end up using the same main units over and over again without any encouragement to explore.
A lot of time can be spent trying to figure out the best team combination (assuming you aren't using guides and what not).

Combat in tactics is more fun and more indepth, some elements in Arc can leave you scratching your head (allies missing a 95% hit chance and enemies scoring critical hits from a 5% hit chance) but others can leave you with a greater appreciation of the game (free hits from gemed allies). But with Tactics it's much more fun and less eye brow raising, if you screw up, you know it's your fault 9/10 and not some cheap mechanic.

I know I'm kicking Arc here but if I was judging this game on it's own legs it would be a 9/10 however Tactics wotl is just more superior in every way (except story :D).

That's my own opinion, I know others will disagree.

How many games can be stored on the PSP-go? or do you have to buy memory cards or something?

I would def. get Persona, Disgaea 2 and Dissidia if possible.

didn't they say it had 16gig in built memory with a memstick drive as well?

GT Portable.

Games nowadays are roughly 1-1.1 GBs, but most older games are 400-800 MBs.

Patapon 2 is probably fairly small 500 MB?

Disgea 2 and Dissidia will be pushing a GB most likely.

This is already available for download on the US store.  You may want to check the EU store.

@ advent Wolf

Games are smaller than you think. Recently released Holy Invasion of Privacy Badman! is only like 100 MB.

You bring up a good point I never thought of before! Games that were previously unreleased in regions for PSP may be available for download, allowing for mass availability. This would be much more fair for countries that were less fortunate when it came to prior game releases.

Arc never got released in EU full stop, I had to import my copy.