It depends on how many plug-in's and synths/virtual instruments you already have.
This is what you
do not get with Studio ...
R-MIX SONAR (Lite Version in Producer)
TH2 Producer (Lite Version in Producer)
BREVERB SONAR (Lite Version in Producer)
Perfect Space Convolution Reverb
LP-64 Multiband compressor and LP-64 EQ
PX-64 Percussion Strip
VX-64 Vocal Strip
VC-64 Vintage Channel
Modular ProChannel
ProChannel Modules:
FX Chains 2.0 ProChannel Modules
Console Emulation
Softube Saturation Knob
QuadCurve EQ
Tube Saturation
PC4K S-Type Bus Compressor
PC76 U-Type Compressor
Plus you get 'Lite' versions of Dim Pro and Rapture as apposed to full versions in Producer.
You can upgrade the Pro Channel with new modules as they come out, but they are proprietary to Sonar, so you will be locked in to Sonar going forward.
You need a really solid system as well. DIY DAW building is not recommended for Sonar.