In-dash units:
Pros
* Look a lot neater
* Integration with stereo system so that audio can cut out when GPS directions are being given or in telephone calls.
* Harder to steal
* Better GPS reception
* Dead-reckoning (ie will still give you position when GPS signal is lost).
* Big screens
* No wires running across the dash.

Cons

* Unit and add-ons are expensive
* Expensive map upgrades
* Limited GPS features
* Some units only have coverage of major cities and highway.
* Can’t take it with you if you don’t travel with your vehicle.
* No regular speed camera updates.
* No regular firmware updates.
* No traffic updates
* Will the GPS unit still be supported in 5-10 years

Portable units:
Pros

* Units are cheaper
* Cheaper map upgrades
* Firmware regularly updated to provide new features or bug fixes
* Can take the GPS with you if you travel.
* Generally have more GPS features
* Most units have regular safety camera updates.
* Map coverage is generally quite good.

Cons

* Smaller screen
* Portability makes them easier to steal.
* Generally can’t integrate with car audio system.
* No dead reckoning, although some units have begun to implement it.

I’m now using Eonon portable GPS which fit me very well. According to your requirement, maybe Eonon portalbe ones also quite suitbale for you.
References :
http://www.eonon.com/GPS-Navigation/E0825.html?utm_source=answers.yahoo.com&utm_medium=answer&utm_campaign=answer