What effects does the choice of companion have on the Inferno by Dante Alighieri?

Inferno tracks Dante's descent through the various circles of hell under the guidance of the Roman poet Virgil. Virgil plays a critical role within the poem—for it should be noted, Dante is not just a passive observer in his travels: as he progresses through hell, he will face various dangers throughout his journey.
This is one of the reasons Virgil is so important: Dante himself does not possess the knowledge or experience to complete this journey on his own. Indeed, Virgil plays a critical role in terms of providing exposition as well as protection and solutions to some of the various obstacles which they run into.
To give some examples, Virgil summons Geryon at the end of canto 16 in order that he might take them on his back and ferry the two poets down further into hell (see Canto 17). Later, in canto 21, Virgil will intervene with a group of demons, requesting safe passage and guidance from them. Through these examples, one can only imagine what troubles Dante might fall into were he to attempt this journey on his own.
In addition, taken from a purely thematic viewpoint, it's important to recognize that Inferno was written during the Late Middle Ages, and (much like the Renaissance which followed it) during this period, Europeans were captivated by the memory and legacy of Ancient Rome. Dante's own work, as a poet of Late Medieval Florence, reflects this fascination, and Inferno itself is filled with references to Classical History and mythology. In fact, one might well say that the hell he envisions fuses Classical pre-Christian influences with Medieval Catholicism. Thus, we see, within Inferno, appearances from the Charon, Minos, the Furies, centaurs, the aforementioned Geryon, Cerberus, etc.
The selection of Virgil then, should not be treated as coincidental. Dante chooses as his guide and mentor the greatest poet of the Roman world as a striking testament to his own poetic legacy. Indeed, in canto 4, Dante finds himself face to face with the great poets of the Ancient World, and the poets themselves include Dante as an equal. In selecting Virgil as his guide, then, Dante seeks to insert himself within this poetic tradition in order to assert his own claim to a legacy within it.