With few exceptions you should go with the lowest client to guide ratio you can afford. A low ratio (a small number of clients per guide) offers several extremely important advantages.
First, is safety. In mountainous terrain a small team moves much faster, avoiding all the bad things you like to avoid, like afternoon thunderstorms, nightfall, exposure to objective hazards, icefall, rockfall, avalanches, hypothermia and others unpleasantness.
On easy peaks and trekking, larger groups may be desirable. These groups offer greater social interaction and the increased party size can give strength to a rescue or other potential problem. But don’t confuse party size with guide to client ratio. Even in a large group, try to go with a low ratio. Two guides with four climbers is a much stronger and flexible group then one guide with five climbers.
Speed and a greater range of available techniques the guide can employ are what makes the small ratio safer than the large.
Small teams generally move faster than large teams. This means you spend less time waiting for others, stay warmer, and have more time to spare at the end of the day, relishing your accomplishments.
Small teams offer greater odds of successfully reaching the summit or completing a tour. A team is only as strong or as fast as its weakest member. The larger the size of the team the greater the odds of someone keeping you back. Stack the odds in your favor and insist on a low ratio.
Small teams also have more inherent flexibility to accommodate adjustments in team composition. Compare a group with 2 guides and 4 clients to another group with 2 guides and 6 clients. If one of the climbers is ill or simply doesn’t have the strength, will or desire to continue, with the 2:1 ratio one guide can take the healthy client, in addition to his or her other clients, on his or her rope and continue to the summit while the other guide heads down with the single client. In the 1:3 ratio, however, the guide going on to the summit may be unable to accommodate the healthy clients from the other rope. In this situation one or perhaps two perfectly capable climbers will be unable to summit because the higher ratios lacked the flexibility to adjust. Smaller ratios generally offer greater rates of summit success.
When shopping for a trip always inquire what the maximum client to guide ratio will be. In general, technical climbs should have a maximum ratio of 2:1, and climbs with a great degree of exposed 3rd and 4th class terrain, such as the Illimani or the Alpamayo should be done at a 1:1 ratio. Low angle glacier climbs can be done at higher ratios, but more than 4 or 5 people (including the guide) on one rope can be a frustrating experience for everyone. 1 or 2 climbers with 1 guide is a good number of a glacier climb.