High tier scanner means more scanning bonus. A +5 scanner will give a scan bonus of +6 (it’s 1+scanningBonus).
High tier harvester means more harvest bonus. Also, for the purpose of figuring out what you can harvest, the level of your harvester caps your skill. (This means you can keep getting xp from grinding low level resources with a low level harvester). However, when it comes to properties, a high HarvestingBonus on your scanner is best. The % chance of an added property is increased by 2x scanning bonus + 2x harvesting bonus. A +5 scanner would give a 12% chance of extra properties (because of the +1).
Base % of an added property is based on a calculated DC derived from the tier of the resource - 10 per tier with a modifier for quality. (e.g. Veldite is Tier 1, so base DC 10). If your harvest skill - not capped by your harvester - is 7 or more levels above the DC, you get a base chance of 45, scaling down to 0 at -7. (The chance is 15% at same level). This chance is improved by half the total harvest bonus on all your gear, not just your harvester. (This bonus is capped at +30%).
So… if you have +10 harvesting gear on, and are carrying a +2 harvester, and scan with a +2 scanner on a resource that is much lower level than you, your total chance is 45 + 0.5*(10+2) + (22+2(2+1)) = or 61%, I think.