I have a Plant/Psi. I say go with that.
I don't think plants and fire mix well.
Plants/Fire has great damage (for a Dom), but little damage mitigation. It will be easier to level than Psi, and it has Consume, which will be easy to use with Seeds of Confusion to keep you at full endurance.
Plants/Psionic takes much longer for the secondary to take off, but once you have Psychic Shockwave at level 38 you'll feel like Captain Awesome of the USS Awesomesauce. The Single Target attacks are also good, but Psychic Shockwave at lv 38 is why people play /Psi doms. Also, there's Drain Psyche, which boosts Regen and recovery.
Both are good, and both will be great Dominators by the time you hit lv 50. The rest is personal preference.
For the record, I play a Plants/Fire Dom. I like it for the even-ish mix of AoE and ST damage and the relative ease I had in getting to lv 50, as well as the very idea of being a walking forest fire. :P
Too many alts to list.
I have to agree with Mr Grump.
Drain psyche and Psy shockwave make the end-game /psi dom better than /fire. I've played both fire/psi (40) and plant/fire (to 50). The latter has become rather lackluster compared to the fire/psi.
Being a walking forest fire though is fun conceptually though.
Malakim
-Playing since COH beta and still love the game!
[ QUOTE ]
Plants/Fire has great damage (for a Dom), but little damage mitigation. It will be easier to level than Psi, and it has Consume, which will be easy to use with Seeds of Confusion to keep you at full endurance.
[/ QUOTE ]... The same can be said for Psi, which has Drain Psyche, at the same level you get Consume, and is a better power. And Plants has plenty of damage mitigation - psi offers more than fire, but plant needs no help at all. I don't disagree with your final assessment, but I do think your opening statement needs work.
plant/fire > plant/psi. -recharge doesn't synergize well with confusion, but high damage does (though it isn't burst in the case of fire). It can work conceptually since some plants can give off tons of heat. I can't find the name of the plant right now (I think it's named something like witch something). I'd still go with plant/thorn though. 
(more burst damage, -def so confused enemies kill each other faster, fast recharging cone, and aim for burst damage). Don't take impale/thorntrops if you do though. Roots is nice later on.
Thnx all, I decided to go with fire. I really dont want to wait till lvl 38 for PSW plus with the buff coming there's a chance PSW may be changed. Thanks anyway..
I have a fire/thorn so i dont want to do thorns again, but it is an awesome set. Cant wait till the buff to see it really shine.
Help me out with this one guys, not sure which to choose so ill leave it in the hands of the posters. This is for PvE.