shimano ultegro gear cassette help
 

I need to purchase a lower gear cassette for a 9 speed as a gift. There are various shimano 105 models and various shimano ultegro models. Can anyone shed light on the differences and which model ultegro is best for climbing hills? Not being a cyclist myself I am confused by the different models of the same brand. All the ones I am looking at are 9 speed and 12-27 (maybe I need a higher number???) but there is the CS-6500, the HG70, the XT, the CS-M760, etc. What features should I be looking for. Thanks.

Remember to take into account the capacity of the derailleur and chainwrap and all that fun stuff. If the bike has a double ring crankset with a short cage rear derailleur, 27t may be the biggest/lowest gear you can go to. If its a triple with a long cage derailleur the sky is the limit. XT is going to be mountain bike gearing, so very low gearing in the 32/34t range. The model numbers aren't important, you just need to find the gear and quality you are looking for, that will work with the current setup. Thought about just doing a giftcard for the amount of an Ultegra cassette to your local bike shop?

Everything on a bicycle works together so they all have to match. If you want to be sure that whatever you buy will work, you need to have a bicycling knowledgable person look at the bike that it's going onto.

The bigger the cog on the back, the easier it will be to climb hills. Shimano rates all of their road derailleurs for a maximum 27 tooth cog. Many new road bikes come equppied with a cassette that has a 25 tooth biggest cog. A 27 tooth cog will be a tiny bit easier for getting up hills but don't expect a huge amount of difference. While it is sometimes possible to "cheat" and get a larger cog to work with a road style rear derailleur if you need an easier hill climb gear than a 27 tooth the safest solution would be to install a new mountain-style rear derailleur at the same time.

You're on the right track, it's ok to get a larger cog.
The posts by "seely" & "retrogrouch" are useful info too.
I combined two cassettes to make a 12x28. Made hills slightly easier, there's not much flat ground where I live. If you want to climb hills all day long, then a mountain rear derailleur with a standard mountain cassette (11x32 or 11x34) is a practical and not expensive answer. You probably have no use an 11 tooth, but maybe you can find a cassette starting with a 12 or even a 13 tooth. They aren't uncommon.
Having a larger rear cog gives you the confidence to climb any hill, even when tired.