WRT stats, I'm not exactly sure what you're going for. Str 18 is excellent but Dex 17 seems excessive if you plan on wearing any kind of real armor. Assuming you'll eventually get gloves of Dexterity +6, there's no advantage to more than a 16 dex if you'll wear a mithral chain shirt and no advantage to more than a 14 dex if you'll wear a mithral breastplate. If you're going for mithral fullplate or banded mail (for the samurai image), you could make do with a 10 or 11. Now, you need dex 13+ for some of your feats so I would recommend simply switching your dex and con scores and going for the mithral breastplate armor.
Equipment:
To get ASAP:
Weapon: +5 Holy adamantium Katana (adamantium to avoid sunders and 3.5e DR, Holy for the same reason, +5 for the attack and damage bonus)
Cloak or vest of Resistance +5: Saving throws are one of your weaknesses cover them as best you can. (If you get a vest of resistance, you can use the cloak slot for a cloak of displacement).
Armor: +5 mithral breastplate
Shield: +5 animated shield
Ring of Protection +5
Amulet of Natural Armor +5
Belt of Giant Strength +6
Gloves of Dexterity +6
Bracers of Health +6
Headband of Intellect +2 (The sooner you get it the sooner you can start spending the additional skill points)
Vis a vis the Ftr 4/Bbn 16 with a greatsword and the same stats:
A. Damage- his weapon does an average of 1.5 points more than yours and when he is raging, he'll gain an additional +3 to hit and +5 to damage.
You, on the other hand will get double your strength bonus to damage. Assuming 30 strength for you, that adds up to an additional 5 points cancelling out his bonus from Rage. You may also have greater weapon specialization which will kick in another two points putting you .5 above him in average damage/hit. He'll still have an attack bonus two points better than you will (even with greater weapon specialization though). On damage, you're comparable but he's slightly ahead.
B. Armor Class. Assuming similar equipment, your AC will be two points better than his when he's raging. You'll also have the option to increase your AC through Expertise--which will probably add 3 or 4 rounds to your life expectancy against a creature like the 3.5e Titan. That will probably enable you to deal more damage over the long run. In a one on one battle you could also use expertise and fight defensively--pushing your AC up to 51 where even his primary attack would have less than a 50% chance of hitting you (before the cloak of displacement is taken into account).
Against weaker monsters where expertise is not significant, you will also have Supreme Cleave which will enable you to take out more bad guys per round. More significantly, it will enable you to take cleave attacks with less risk. The barbarian may often find himself in a situation where he needs to put himself into a flank or other disadvantageous situation in order for it to be possible for him to cleave. You, OTOH can stay out of the flank and five foot step in order to cleave.
C. Other Defense: The fighter/barbarian will have Uncanny Dodge. You, OTOH, will have a much easier time avoiding AoOs. You also have the potential to get Blindfight, Close Quarters Fighting, Iron Will, and other defensive feats at a much lower opportunity cost than the barbarian.
D. Options: Due to your greater number of feats, you will have a lot more options in a fight and, in a situation where mounted combat is an option, you will be much better than the fighter/barbarian.
