many factors
Having just purchased a new headsail I just finished making this decision about 2 months 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 and the more the sail stretches, the faster it will lose its forward driving shape and develop more heeling shape(it will get "blown out"). 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 weight 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.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 was completely amazed by the difference the new sail has made in my boats performance. The boat heels less now with the flat 145 than it did with the old dacron 110(blown out)--far less. I have reefed the headsail 4 times- twice to a 130 and once to a 110. The foam luff maintains the sails flatness very well. But the efficiency is obviously decreased as the leading edge(the furled sail)gets larger--especially when furled to 110. No genoa can compare with the power that an assymetrical spinnaker can produce on reaches and runs. I can hold my chute up to about 60 degrees apparent in lighter wind conditions. Any more wind and she is overpowered. Can hold the chute in mod wind on both beam and broad reaches. An invaluable and very fun sail. I would recommend buying this first. I would also recommend getting a snuffer sock for the chute as well as the ATN tacker so you can raise and lower the tack of the chute--on the deck for close reaching and raise it for broad reaches. You can live with your old genny for now as it probably only performs poorly while pointing and buy the new RF genoa at a later date.