I am not an expert, but there is a list and so I will go throw it. For REAPER.
I put my comments directly into quotas, for readability.
I start from the features which have better implementation in REAPER, compare to Sonar:
track folders
combined midi/audio tracks (make instrument track)
patch points, aux tracks
In Reaper, a track is everything: audio, midi, video, aux, bus, folder, VCA. By default, folder is also a bus, but that can be decoupled. Folders support sub-folders (any depth). Tracks can be but into multiple groups, as master/slave/VCA for arbitrary set of parameters.
MIDI/Audio routing is homogeneous (unlike in Sonar, MIDI also can be routed as "Send"). Routing Matrix gives one picture overview of the whole routing, with "swipe" gestures.
There is "IN-FX" bin, so plug-ins (MIDI and Audio) can be applied before recording (especially useful for MIDI manipulations).
Sub-project is an item which can be used as usual media file (automatically kept in sync).
MIDI effects (mfx)
CAL
EEL2/Lua/Python/C++. For MIDI, audio and the DAW behavior. Build-in editor for scripts.
Scripts can work as "one time processor" (CAL direction) or as FX processors (MFX direction).
Big set of (M)FX scripts is bundled. Huge set is in SWS extension (almost standard to have in Reaper). And most users publish there own...
That is the list of features with similar implementation
unlimited I/O, audio/MIDI tracks, busses and VSTs
opening 32bit VST in 64bit
resonably good VST handling
ability to switch off automatic VST scan during starup - It seems like there is some VST scanning at startup, but it is quick. I mean when Sonar does something longer when its auto-scanning is on.
plugin load balancing - Not tweaked CPU handling in Reaper is efficient. May be under condition when Sonar plug-in load balancing as such make sense (note that is very specific case, as mentioned in Sonar docs) Reaper can saturate some core, but I have no such conditions. So for me (and some other users) ASIO buffer size can be kept low longer then in Sonar.
dynamic control bar - multiple arbitrary configurable tool bars
track templates, screensets
convenient interleave (mono/stereo), phase switch and input echo on/off switch on every track - Reaper has a bit different approach for echo on/off, for some use cases it is better for other worse.
quick group and smart swipe - multiple independent persistent groups and quick group for tracks and items
expandable fx & sends (in Inspector/Console view) - no build-in inspector... but Console view can show as many fxes and sends as the screen allows.
dim/exclusive solo - called "Solo in front"
copy entire clips as linked clips (siblings) - called "MIDI pooling"
MIDI events chase on play
video handling - I have not tried, but it is there
non-destructive audio & MIDI editing
Ripple editing
take lanes & speed comping
clips auto-crossfade
bounce to clip(s) - called "Glue"
audiosnap - transient-based audio editing. A bit different, but there.
clip FX rack (clip based fx)
clip/track: apply effects (process in-place)
freeze/unfreeze track
groove-clip looping functionality
rxp file handling - convert or slice, no build-in direct handling (in Sonar there is only convert).
export selection to audio - select clips/tracks and time region and export (bounce) to audio file
MIDI view/editing directly in Clip pane
step recording (MIDI)
quantize + swing ability
Length/velocity scale
event list, markers and tempo editing
Not yet, but almost there
Melodyne - ARA support. Reaper is waiting for ARA2 which is coming soon (short video of it in Reaper already exists)
Different (better or worse is subjective)
Skylight interface - Many newcomers do not like Reper interface. But there are many themes and they can include much more tweaks then in Sonar themes, I mean not only images and colors can be changed but also the structure.
smart tool - Reaper has configurable mouse behavior, but not all Sonar smart tool gestures can be done
mix recall - there is SWS Snapshot extension. I have not sufficiently used mix recall / this extension to compare.
Snap To/Snap By switch - I have not found snap by. But setting snap offset is a bit easier then in Sonar, so I compensate with it (and I have found that less error prone).
radius algo for pitch/strech audio - as many other DAWs, Reaper use elastique. You can find many comparisons, some say izotope is better.
step sequencer - no buil-in. There are some bundled plug-ins (MIDI sequencer), seems like can do some sequencing.
find/change (interpolate), select by filter - find change can be done by plug-ins(scripts), so not destructive. "Select" is a part of Filter in the MIDI editor.
FX Chains - FX chains can be saved, but they are "flattened" when inserted into the bin.
I do not know / not found yet
X-Ray feature
groove quantize
save as MIDI format 0 and 1 - Reaper can definitively save MIDI files, but I have not tested how
SysEx
Not there
matrix view - there is PlayTime. Not free, looks a big ugly and behave quite ugly, but support usual for matrixes controllers.
Not and will not be there (and not in any other DAW)
drumreplacer
vocalsync
ProChannel
QuadCurve EQ - (Reaper has bundled EQ with spectrum analyzer)
Tape Emulator
Tupe Saturation
Blue Tubes suite (CW version works in Sonar only)
Adaptive limiter (locked to Sonar)
Truepianos (CW version works in Sonar only)
The following works in any DAW. Till DX in DAWs without native support, but there is DX->VST plug-in.
Reaper support DX plug-ins natively.
sfz file handling - that is for synth, not DAWs. Corresponding CW synthes are not locked.
Sonitus suite - Compressor and Gate without side chaining
L-Phase plugins
AD2/Session Drummer 3
Cakewalk TTS-1 - DX
Lounge Lizard - AAS offer unlock codes
z3ta+/Dimension Pro/Rapture
Tone2 Bifilter
Breverb Sonar, Rematrix Solo and TH3 Sonar - Overloud offer unlock codes