The Parry Chance for Bushido Formula:
When using Two-Handed Weapon, No Shield: Parry*Bushido / 41,140 = Parry Chance
When using One-Handed Weapon, No Shield: Parry*Bushido / 48,000 = Parry Chance
When using One-Handed Weapon, w/ Shield: (Parry - Bushido) / 4
In the example you provided, (300 parry, 100 Bushido and assuming the player is using a two handed melee weapon) the player would have a 73% parry chance.
Although it is possible for a Bushido/Parry Player to achieve a higher base parry chance than a player with a shield, some things need to be taken into account. There are Pro's and Con's to the path of Bushido, and I will do my best to point these out:
Pro's
Higher base Parry Chance, when using a two-handed melee weapon without a shield
Decent base Parry Chance when using a one handed melee weapon without a shield
Ability to use Evasion, Confidence, Lightning Strike and Counter Attack (All great skills, but with limited application in PvP. Evasion is the most desirable, but comes with the price of a 20 second cool-down)
Con's
Sacrifice the use of a shield entirely (This means giving up the HCI, SSI, and DCI the Mirror Shield Provides)
Inability to defend when using a bow (like how some guys rock the one handed bow and the Mirror Shield? Forget about doing that.)
When Disarmed, Defense becomes greatly reduced. (This can be catastrophic in both the field and duels, while a Parry/Shield Counterpart still has a Parry Chance when disarmed)
Expensive and difficult to "Complete" gear. Requiring many sockets to produce capped HCI, SSI, and DCI
I hope this information helps, and if you have any questions regarding Bushido, I'd be happy to help. I am no expert, but I've got a bit of experience.
(PS- Kane, I hope I manged to articulate my stance on Bushido/Parry here in a more effective way than I did drunkenly last night.)