Most people find that a portable aerial won't produce a strong enough signal for Freeview but, if you live close enough to the transmitter, it might work.
Also, many people find that a loft aerial isn't good enough for Freeview. It's often necessary to upgrade to an external aerial.
Part of the problem is that most digital transmitters are currently operating on reduced power. They can't be used at full power (because of interference problems) until the digital switchover is complete and the analogue transmitters are turned off.
There can be no clear-cut answer to your question. Ask around the neighbours to find out what type of reception they get. Then try the loft aerial. If it's not good enough, you know that you need an external aerial. If the TV's tuner locks on to all of the digital channels straight away, on the first scan, it might be worth trying a portable aerial.
Chris