250w vs 400w in a 60cm by 60cm by 140cm tent
Total differnce in electricity cost per month due to bulb size needs to be a deciding factor & who pays the bills etc.
Heat issue will need to be addressed with
suitable ventilation, to much heat can be deprimental to total yield.
How much do you want to spend on kit ? cost do add up once you start fine tuning your grow... improvements/adjustments can be made later.
Its an expensive hobby & the seeds are not cheap
Right lets get down to some basic's
250w in a tent of 60cm by 60cm is fine, ya still looking at 60w per square foot, thats pretty good. I would certianly go as far as 4 inch/100mm extractor fan to vent your tent tho, air change is quite important thing in growing & a fan that size should keep all in check temperature wise.
Their might be some temp problems in summer months tho as this may effect ambient indoor temps along with central heating also
If you do decide to go down the 400w route, well ye results might be better if the temperaure was controlled suitable, once you factor in size of air cooled reflector/reducers/ducting thats gonna be a pretty tight fit in a tent that size, next to impossible...
Which leads to getting a Bigger 'n' better tent
Please NOTE that not all grow tents are suitable for air cooled reflectors.