My advice as far as what to get now would be to figure out what part of your game needs improvement most. Example, would more length off the tee help you the most, or are you missing too many easy putts, or you need to be closer to the pin on your short shots. Then you can decide if a putter, driver or wedges would be what you need right now.
If you only have enough credits to buy just one item, decide where you think it would help the most the quickest.
Or, just hold off for now, work your way up in levels save up some credits and go for a complete set of clubs. With a complete set not only will you get a good enough upgrade in equipment, you'll also have the same meter speed across all clubs.