A 928 can be a nightmare to maintain. These were very expensive cars when new and need to maintained as such. CIS, Electrical gremlins, general scarcity are all issue with those cars. Now, some people love them to death and wouldn't want anything else (we call them crazies
) They are cheap for a reason.. much like 944s.. when you realize how many are poorly maintained and how much it takes to make them right, people just dump them.
911's are actually fairly easy to maintain because they are very hard to break! A '78-89 vintage 911 is very robust. Also be aware that most 911s are really $20K cars, no matter their asking price. Need a new clutch? theres $2k. Need a motor rebuild? There's another $6-15K.
You need to spend the time to find, again, a well maintained example.
Also
1) Will either of these be daily drivers or merely weekend cars for you
2) Do you intend to do anything like Autocross or DE events?
3) How mechanically adept are you?
4) Is $10K the entire budget? For any 20+ year old car, I would want at least $2-3K of available fund for the "oh shoot, no one caught that" issue that will come up.
While it pains me to say it (I am a 911/914 guy), Perhaps you should look for again, a well maintained 944 model. They can be had in the $4-8K range which will leave you money left over to address any short commings, except, perhaps horsepower.. (ie $1500 for waterpump/timing belt; $2k for a new clutch etc).
More importantly, spend time driving some examples. Even the $20K 911s you know you can't afford, simply to learn what a nicer one "should" feel like.
It is possible to find that diamond in the rough.. but it will take time to do so.