Hey Dania
Getting images in TIFF is a great idea. TIFF is different in raw in that TIFF can be viewed in all kinds of computers (so any Windows PC has the ability to view a TIFF file) but this is not the case with a RAW file - RAWS are specific to the camera they are taking with so if you take a Canon RAW image and then view it on a PC that doesn't have the right Canon software you will not be able to see it.
TIFFS are just as pure as RAW's though when it comes to image size and quality (jpegs are always compressed). Photographers may try to put you off because TIFF files are huge so take take to provess nad burn onto cd for you and take them longer to deal with than jpegs. They also take up more memory space. It's not a HUGE deal, but you would need to check that the photographer is shooting in RAW or TIFF to begin with. Converting a jpeg to TIFF is pointless because the image was compressed at the size of shooting. However, converting RAW to TIFF or shooting in TIFF means all the orginal quality was kept so you need to know if the photographer is using these formats to start with. If so, just say you are happy to take a DVD with all the TIFFS and you will sort them out yourself at home. It's an extra 15-20 mins for a competent photographer!
Hope that helps!