Like you I wanted the option to pull the carputer. For about 60 days in the winter (my situation is the revese of yours) Jan/Feb up here can see temps
-30c and depending on how my system reacts I have the option of pulling it out of the car in about 2 mins.
If you go H/U try and get a digital In (aux) if possible.
Although a carputer is great, the option of not starting it up all the time on short trips for lunch, a quick run to the store is a bonus. Also, if a big job to add to your carputer comes around you can pull it out of the car and work on it for several days with MP3's, FM still running in your car.
There have been times where something has failed, and have to send it away under warranty.. it can take a month, so glad to have the H/U.
Also depending on the unit you get and how much you are into sound, a good H/U will give you options to adjust your sound stage correctly,precise and on the fly. ie: time delays, X-over frequencies/slopes and PEQ. Oh and the VOLUME button one source very fast reaction for volume and probably mute
Voltage to your RCA's going to your amp(s) is also a concern. Some sound cards will support less than 2v which can introduce noise. My H/U runs 8v to my amps which helps in absolute 0 nosie on the line.
2008 350z GT Installed since April 22nd 2011 - Worklog 2000 Protege Installed Since April 2, 2005 - Intel D201GLY|Fusion Brain|ODBPros ODBII|Engenius|GPS Rikaline|Powermate|Motorized Lilli