Whatever your means of transport, you are likely to find that the main problem when moving around Spain in particular is centralization. There is an improbable bureaucratic streak in the Iberian personality which tries to create order out of chaos, and usually fails, but is quite likely to succeed in making life more difficult for everyone. This is evident in the road system, the railway system, bus routes and many more things affecting travellers.
A road map of Spain looks a bit like a misshapen spider's web, misshapen because the strands going out from Madrid in the centre are all in place, but most of the ones which should encircle the centre are missing. Look at a road map of any province and you will see a similar pattern. Road planners have traditionally supposed that Madrid is the centre of Spain, therefore everyone wants to go there, and that the capital of each province is the centre of that province and so everyone will want to go there as well. This can mean that to get from A in one province to B only a few kilometres into the next province, you have to travel to both provincial capitals, which can be frustrating and time-consuming.
Many of the main intercity routes are covered by toll roads, which are avoided by most drivers, leading to overcrowding on the main roads parallel to them. While Spanish roads are generally quite good, Portuguese ones are much less so, and in both countries, they are much narrower than you are probably used to, though extra lanes for overtaking, especially on uphill stretches, are now the norm rather than the exception. Great care should be taken in Spain, they are not considerate drivers. The combination of poor roads and reckless drivers can make motoring in Portugal, especially, a really hair-raising experience. It is a good idea to familiarize yourself with traffic norms, which may be different from what you are used to: overtaking more than one vehicle at a time, for example, is illegal, and you are obliged by law to carry certain things such as a red warning triangle to be placed a few metres behind your vehicle in case of breakdown. Traffic police theoretically have the right to impose on-the-spot fines: fortunately for you, they often prefer to overlook offences committed by foreigners to avoid the paperwork, but it would be silly of you to depend on that.
If you are renting a car in, say, Lisbon and want to leave it in Barcelona, be very sure that your car rental agreement allows this, because it is more likely than not that the car hire company does not allow cross-border drop-off. Also, if your pick-up is not going to be at the airport and your hire company says that they will be glad to deliver to your doorstep, for the second week of your holiday for example, do not take their word for it. Doorstep delivery is the exception rather than the rule. Use your Spain cell phone rental to rent a car.
The same overcentralization problem applies to railways as to roads, with the added difficulty that the railway system has never really been well developed in Spain, and it is only recently that train travel over any distance has become bearable. The new AVE high-speed trains being progressively introduced will make a difference, but this is not happening quickly, and for long-distance journeys within Spain and Portugal you may find flying more acceptable - after all, if you only have ten days, it is frustrating to have to spend one of them on the train to Barcelona. Use your international mobile phone rental to make a train reservation.
A much better choice than railways for most long-distance travel, intercity buses are frequent, comfortable and in most cases a lot faster than the train. In addition, long-distance buses still operate overnight, so you can often save on a night's hotel stay. However, the bus sector is extremely fragmented, some operators only being active in a single province. So if your starting point and destination are not capital cities, you may find the centralization problem to be even worse than with the railway.
An enticing option in many parts of Spain, particularly for travelling one of the great routes such as the Camino de Santiago, cycling is also becoming steadily more popular with Spaniardsthemselves. Cycle lanes and paths, for example, are still rare, but much less so. Unfortunately, cycling in Spain and Portugal has a couple of big drawbacks. The most serious is that it is frankly more dangerous than in most of the rest of Europe, due to Iberian drivers' complete insensitivity towards cyclists. The Portuguese and Spaniards are extremely discorteous when behind the wheel, and this lack of consideration leads them to overtake cyclists at breathtakingly narrow distances, close in on them when pulling back over, nuzzle up to their rear wheels, and so on. If cycling, avoid travelling after dark altogether, and take every precaution possible. Use a helmet and reflective or fluorescent acessories, and do not be aggressive: you will not force cars to give you more space by refusing to budge. Use your international cellular phone rental to get more cycling info.