Every ingredient, water, grain, hops, yeast, plus process affect the end product. Never underestimate any element. A perfect recipe with poor sanitation and process will turn out bad. Using the wrong yeast, or the right yeast at wrong temperatures will make a big difference in the end product.
Marris otter is a pale british base malt. Vienna malt is continental malt that is kilned at a slightly higher temp to create a slightly darker color and toasted flavors.
2-row barley has a higher starch/husk ratio and is relatively low in protein and enzymes. The result is an easy to mash malt with higher gravity contribution per pound.
6-row barley has relatively more proteins and enzymes. It is great when mashing with non-barley grain adjuncts that are low in enzymes. Because it has more protein, it usually requires a protein rest in the mash to improve clarity in the final beer.