Here are some general thoughts.  I'm sure others will provide more specifics.

In general as the horsepower increase the voltage of the most-economic (motor purchase cost) motor increases.

Larger cables are required to carry higher current for the lower voltages.  (a big consideration if motors are far from source)

I would think the breakers for the lower voltage choice might be more expensive since more current must be interrupted for same fault mva.  (Then again, fault mva might be somewhat lower at the lower volt system.. depending on your system).

Suggestion: Reference:
1. Robert W. Smeaton, "Switchgear and Control Handbook," Second Edition, Mc-Graw-Hill Book Co., 1987
Reference 1 Table 3B "Standard Voltages for Large Polyphase Induction Motors" indicates:
Voltage [V], 60Hz:                       HP
460 or 575                                 100-600
2300                                          200-4000
4000 or 4600                              400-7000
6600                                           1000-12000
13200                                         3500-25000
Suggestion: 50Hz will require to reduce those HPs somewhat
M.G. Say, Alternating Current Machine book indicates lower limits of ratings that may typically be (50Hz):
150kW for 3.3kV
300kW for 6.6kV
1MW for 11kV
Definitely, the HP will be decisive in an economical voltage level selection.

Hi friend ,

I feel 6.6kv wil be optimal for DOL START what is your power transformer rating.you have to do voltage drop study before going for this option

When you have 6,6kv supply 3.3 kv is not advisable because you have breakers in thr market most are 11 kv