I don't feel I can truly vote for either one. The decision is ultimately yours. I can offer some pros & cons for each car to maybe help your decision. And maybe if I do that some others will either respond as well or may even do some actual voting in your survey box.
The 2008 : A fully loaded touring (if it actually is a touring & the dealer is not lying to you about it just to make more profit for himself) is very desirable with the better suspension & bigger wheels. The interior is not as confining/claustrophobic as in the 2010. You have 4 easily accessible cup holders on the center console (2 front & 2 rear), 2 more cup holders combined with spacious map pockets in the 2 front doors, a "secret" storage drawer that slides out of the front of the center console. A proven track record of dependability [in production in the same configuration from 2004 thru 2009 & many thousands produced (therefore many wrecks out there to cannibalize cheaper parts from should the need arise & new dealer parts are too expensive for you)]. There should be some of the original factory warranty left (unless there's more than 36,000 miles on it) for you to buy an extended warranty (if you so desire), but don't pay more than $1,000 for a 7-yr/100,000 mile/$0 deductible one (you can get one from Troy/Warranty Shack for that amount---I don't think there is any info readily available about that here on this site, but there is over at PriusChat.com ). However, if the mileage is over 36,000 & you would still like to get an extended warranty, it should still be available from him as long as the car is a "Certified" used car.
Only 2 drawbacks I can think of are the fact that there is no EV mode to force/keep the car in electric/battery mode if you only need to move the car a short distance & don't wish the gas engine (ICE) to start. There have been instructions posted on how to enable that (as the non-USA versions of the same vintage were so equipped) by installing your own switch & splicing into certain wires, but that will void any USA warranty if you do that. The other big issue for you may be the fact that the fully loaded cars have the HID headlights (if the car has fog lights in the lower grill, then it has HID's) & the HID's only since 2006 have had a lot of problems burning out prematurely & being very expensive to replace [bulbs range in price from $150-$300 apiece (cheaper on line but questionable quality) & about $400 labor to remove the bumper for access (not always necessary if you or a friend has small hands). As a work-around, halogens are NOT interchangeable due to the system (ballast & high beam shutter), unless you replace the complete headlight assembly (lens, reflector back, etc).
Now for the 2010 : Basically an all-new car (with the exception of the Hybrid Synergy Drive) with the typical teething problems of a new model [dash rattles, braking issues (being addressed by a re-programming recall), etc)]. The engine is different (& more fuel efficient), the transmission has been improved (by the removal of the internal chain, replacing it with a gear), gone is the rubber bladder that caused inconsistent fills of the fuel tank before, the headlights are now a combination of halogens & LED's (NO HID's), an EV mode is now incorporated/standard. I'm not sure if they still use the coolant thermos storage system any more, but I do know they no longer have a belt-driven coolant pump [they're all electric motor driven now---in the older generation there was a coolant pump (one of 3) that was driven by a serpentine belt that needed occasional replacement]. BY THE WAY : there are NO timing belts in the Prius engines (2004 thru 2010---not sure about the 2001-2003)---just good sturdy chains. There may be more pros, but I don't own a 2010 (I've just driven a few of them for only 5 minutes at a time at the car auction I work for). You have the remainder of the factory warranty (unless somebody put more than 36,000 miles on in less than one year). The same applies here as with the 2008 in regards to the extended warranty & if it is a "Certified" car.
The drawbacks I see are the claustrophobic/confined areas of the front seating (intrusive "bridge" between the dash & center console), lack of map pockets in the front doors (only small cup holders) however there is a limited storage area under the "bridge", only one cup holder on that center console (& only usable if you slide the console cover/arm rest back---an awkward position for resting your arm when in that position), & if I am correct on this, no cup holders usable in the rear unless you fold down the center arm rest (reducing rear seat utility & comfort somewhat). I'm not sure if the Prius II comes with the backup camera or not (I think one of the 5 models does NOT have it). The price you will pay for the 2010 will likely be a lot more than for the 2008 & there are fewer 2010 wrecks out there as of now to cannibalize cheaper parts from (should the need arise).
I hope I haven't confused you by all this & I hope others will now speak up to offer their advice (& to correct any mis-information I may have given).
Hey, try this link to help you determine if the 2008 is indeed a "touring" version, but be careful & scroll down slowly through the model years so you don't go past the 2009's & get into the Canadian or UK (& other foreign countries too) models (that will really confuse you).