This is a rather tough question because there are so many variables to consider before you can make an informed decision. You'll want to figure out which is more cost effective, the amount of time that could be gained or lost by choosing a particular form of transportation, whether or not there could be hassles associated with each, and of course, the relative availability of these types of transport. And each of these concerns will vary by the region you are visiting (amongst other things). So here are just a few issues to tackle before you hop a plane and head for your destination.
The first thing to consider is the cost that comes with different forms of transportation. In most cases, renting a car will be the more expensive option, not only because of the sticker price, but also due to the added costs of fuel,
insurance, and so on. In general, taking buses, trams, and trains of various sorts is bound to be less expensive. But when these forms of mass transit are not available you may find yourself stuck shelling out for pricy cabs or other modes of transport, which could quickly close the gap in cost. And if you're traveling for business, the time-is-money rule may also apply, in which case convenience could also play a major role.
For example, consider that you may find yourself having to take several trains or buses to get where you're going. Each one no doubt entails some time spent waiting and if there are delays or you miss a leg your wait just got a lot longer. Some cities, like Portland for example, have exceptional systems of mass transit that are reliable and widespread. But in many locales the public transit has finite schedules, stops, and hours of operation, which could make using them somewhat difficult. In this case, having total control of your own mobility is of paramount importance and a rental car can provide for that.
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On the other hand, driving around a strange city is likely to come with some headaches. You could face all kinds of setbacks dealing with unknown traffic schedules and problem areas that the locals know to avoid. Of you may not entirely understand the rules and roadway etiquette that govern driving, even within different regions of your native land (much less foreign locales). In this case, having an experienced party on hand to transport you could be a blessing.
Finally, you may face availability issues in either area. For example, you might have to deal with buses that don't run after certain hours of the night (leaving you searching for a cab). Or you might not be able to secure a rental car large enough for your family in countries where smaller vehicles reign supreme (don't expect to find used GMC trucks or Ford SUVs on the lot). The point is that there are a plethora of potential pitfalls facing the traveler that fails to account for such possibilities during travel. So do your homework ahead of time and weigh your needs versus available options. That way you'll be much more likely to find that transportation that suits you best every time you travel.