General comments on log sampling rate.
A more frequent rate will show your dive profile in more detail. It allows you to observe depth changes in greater detail. Changes which, and/or the extent of which, someone may not be fully aware of during the dive. This information can aid in improving buoyancy control and developing safe dive habits by providing feedback that can be calmly analyzed at a later time. Greater detail is also useful in observing how different conditions affect our dive. Calm, surge, currents, up/down swells.
Someone not fairly competent in buoyancy control and safe dive habits, no yoyo up/down bouncing, may be better off with a faster sampling rate. See more, observe improvement by comparing samples as one progresses. You don't fall in that category, give a try anyways. You never know what you're going to see.
Bottom line there is no best sample rate. Try different ones, use whichever one you like the best after considering your need to view detail with your need to store X number of dives.
Answer to your second question is yes. More detail takes more memory space leaving less available memory for dive storage. Maximum stored dives is 255 on slowest rate. Thats a lot of diving. What dive length was used to reach this figure? How do faster settings affect it? Read manual and experiment. Hopefully someone else can help you here.