It really depends on your budget and the length of time you have. The 4nt on land and then 7nt cruise is one of the most popular. But you could do 5/7, 6/7, 7/7. You could do 1nt Fairbanks, 2 Denali, 1 Anchorage, or you could do 2nt Fairbanks, 2 Denali, or you could do 2 Denali, 2 Talkeetna. There are way too many combinations of cruisetours. It would benefit you to sit down with a cruise oriented Travel Agent and look over all your options. Some have more coach travel, some have more train
travel, some a combination of both.
By doing the land first you get all your unpacking/packing, and moving from destination to destination out of the way first. Then you can unpack once more on the ship. Either way you do the cruisetour will be fine though. Your probably going to have a busy scedule site seeing no matter which way you do it.
The last time I went to Alaska was in the month of August and the weather was wonderful. Alaska weather can be unpredictable so prepare for all types of weather.