several considerations
Having just purchased a new headsail a just finished making this decision about 2-3 weeks ago. The first thing you must consider is how strong is the wind in your area. If the average is 14 or below I would definately get a 155--the most sail you can have without being penalized. Having said that, a few qualifiers---1)First and foremost you need a sail with minimal stretch since more sail cloth(155 -vs-110) equals more stretch(will lose its forward driving shape and develop more heeling shape) and you need a flat sail on the 28.5 which is somewhat tender. I went with a carbon fiber tape drive from UK sails which was about 900 bucks more than a normal dacron RF sail, for this reason. Also, carbon fiber tapes lend themselves very well to the RF application since the main enemy of carbon tapes is folding(carbon is impervious to UV) and you will never be folding the sail, only rolling. The carbon tapes will never stretch, thus, the sail will hold its shape to the end. Also, I put a light weight tafetta on the side opposite the carbon tapes to aid in durability which adds only minimal shape to the sail and I do not have many light air days anyway.2)Secondly, I had the sailmaker cut the sail aboout 10% flatter than usual to reduce the amount of heeling in heavier wind conditions, thus extending my wind range before having to reef the sail. Remember that a reefed headsail(despite the foam luff) is far less efficient than a non-reefed sail of equal size. The foam luff helps retain some flatness, but the foil shape that the sailmaker cuts into the sail is rolled up when the sail is reefed only 20%. Having the sail cut a little flatter will decrease the power of the sail in lighter air, but by decreasing tension on the backstay I can add some draft. Having said all this, I went with a 145 on my 28.5 since wind here averages 15-20 and also because I have a full batten main with tapered battens and didn't want to crowd the slot. I have sailed with the sail only twice. today the true wind was 16 knots and the boat was well balanced (12 degrees of heel) with no reef in either sail. I think in heavier air I will try reefing the main first but have not had the opportunity to try this yet. Also, I think moving the lead to the toe rail my also help in depowering. My boat is deep draft but I don't really think that matters in you decision--what's good is good. Hope this helps a little, I know it is a tough decision. Bottom line is to speak with several lofts, find the guys that help the most and go for it after careful planning.