I too had the same question a while back. I did a good bit of research and though it seems NiMH's are making a good showing, it still seems as though the experienced pilots are sticking with the NiCd's. Many of the touring pilots use a variety of battery packs, but then they also use regulators and redundancy packs as well. There can be more to this subject than what appears just on the surface of the topic. When I was asking about this I asked several guys at the field. Many have years of experience in building and are considered top notch builders. These guys have 25 -30 years experience and up. Hands down the majority are still using NiCd's. Maybe it's a reluctance to change. Maybe it's just the old school vs the new school. There have been several post on this subject and much info can be gained by doing a search on the subject. In the end you will have to make your own decision because you will get gobs of info supporting the old vs the new.

As to capacity. I used a 600 mah in my trainer and a 40 sized stick. Now mostly use 1100 mah NiCd's in most of my other larger airplanes. I have a 1700 mah in a Pitts with a gas engine. Keeping your battery charged with a charger at roughly 10% of the capacity is a good rule of thumb. Stock kit chargers charge at 50 mah will never get a higher capacity battery up to a full charge. Also want to cycle about every 3 months and it's a good idea to change batteries at a 3 year interval. When cycling you can see if the battery is loosing charging ability if you have a charger that tells you how many mah's was received when cycling, so the three year interval can be less, but many pilots won't use a battery for more than three years. This is a wide open subject with much information to be learned. I am still learning.................. Good luck and have fun.