Clearly this is an electronics forum, with many comments on circuit configuration, loop stability and the like, and only one comment on actual thermal modeling of the environment, plus Kleinstein's comment on heat distribution. Admittedly thermal modelling software is every bit as complicated as EDA software, and generally just as pricey so most of us have little access to it, but that isn't an excuse for ignoring key parts of the problem.
There needs to be attention paid to the fundamentals of the problem. Which elements (if any) need to be isothermal? Which need to be stable over time, but not necessarily isothermal? What are heat paths to and from those elements? What are the heat capacities and thermal conductivity of those pieces? What are the overall control loops, including those elements? Note that making heater power much greater than heat flow out makes control inherently non-linear and complicates stabilization. This is one advantage of TEC as you can get better balance to these flows.
For a reference oven, where there is much less time pressure to reach the operating temperature, no variation in the thermal characteristics of the controlled item, and little or no need to vary that temperature the inherent reliability of a mechanical solution (insulation, nested boxes etc) combined with a really simple outer control loop will be the best choice. Very different case than an environmental test oven.