Bag work or sparring? For bag and pad work, either would be fine. Some people even go lighter.
For sparring, 16oz gloves tend to be the rule of thumb for most people. There are also some schools/areas where 16 oz gloves are mandatory for insurance purposes. You should check with where you're training for that info.
What's your budget and where are you located?
These gloves are often recommended for the budget minded and/or those just starting out. Good protection, good quality, comfortable, and only $42 a pair. Shipping through the US is free (not sure about international).
http://kofightgear.com/sparring_gloves.htm
Comes in both 12 and 16 oz weights, so you can grab one of each and keep the 16's just for sparring.
The reason for that is that the foam padding used in sparring gloves isn't as dense as the padding used in bag gloves, and using sparring gloves on the bag will cause the gloves to wear out more quickly, causing the padding to compress and not absorb the impact as well, potentially causing greater discomfort for your sparring partner.