Yak to Sukhoi comparison.  by Alan Cassidy British team and team trainer many times!

I flew the Yak 55M for three seasons and in two AWACs, and have also done 3 seasons in the Su-26 and flew WAC96 in a Su-31.
I've accumulated about 150hrs of Yak-55 time and about 175 hours of Sukhoi time, including quite a bit on the -29.
Not to boast - I have been fortunate indeed - but to show I have some relevant experience.

It is sensible to make direct comparisons only between the single seaters.
There are two things to compare:

"Performance" in terms of power/weight ratio, thrust and drag, and
"handling" in terms of controllability, and stability.

Performance:
They share the same engine, and you can fit Russian or MT props, so it is pointless to take a big stand either way on power/thrust. Although the 3-bladed MTs theoretically produce more thrust at low airspeeds than the Russian paddle props, in two years of flight testing our Su-26, I got better climb figures from the latter. So, allowing for variable technique, there is probably not a lot to be argued about there either, except costs and the ease of hand-starting a 2-blader when the air runs out.

Under Experimental rules, you are able in the US to add or remove systems without a lot of paperwork. So you can heavy-up or lighten your plane a fair bit. Given that the basic weight of a Su-26 is only a bit less than a Yak-55, there is also not much consistent difference in power/weight ratio.

The only performance parameter left is drag - and this is where the big difference really comes.
The Yak seems to have more form drag and certainly induces a lot more drag going round corners than the Su-26/31. Maximum level speed favors the Sukhoi by around 20-30kph.
Diving a Yak to get a very high speed - often the case for figure 1 - say 420kph, takes an inordinate amount of height. Perhaps 3,000 feet down in a 60 degree dive!
Through a sequence, energy management is much more difficult on the Yak and anyone inexperienced enough to make a long down line and then pull 8g will suddenly find their soaring Eagle has become a flapping chicken. Consequently, the Yak-55 is really only just capable of flying Unlimited (the Unknowns certainly present problems) and demands more gentle cornering than the single seat Sukhois.

At the Kansas AWAC with 3-4,000 foot denity altitudes, some pilots were even having problems sustaining the Yak's energy through their own-design 300K sequences. What price 420K?

Conclusion: If you want to stand a chance of winning st Unlimited, you better get a Sukhoi not a Yak.

Handling:

The Yak has pretty conventional stability and reasonably light stick forces. It is relatively natural to fly for the less experienced. You can roll it either way, full deflection with one hand, unlike its predecessor (Yak-50).

The one feature of the Yak's controls which presents difficulties is the sheer mass of the huge metal ailerons. They have inertia and making quick aileron reversals, as in a hesitation roll, takes a lot of effort and is best done with two hands. I must emphasize that this is not because the aerodynamic forces are heavy, just because of the aileron inertia.

Although it is capable of great tumbles, because of the proximity of the pilot and engine to each other, the Yak is a most docile and forgiving beast when man-handled and has never given me a nasty shock. It is a kiddy-car to land.

The design philosophy for the Su-26 was vastly different.
It was built very close to neutral pitch stability. It has immense pitch sensitivity and responsiveness. This enables fantastic corners and very rapid onset of very high g forces.
It is not unusual for a first-timer, especially an S2B driver, to hit 10 or 11g on their very first pull of the elevator - if they have enough airspeed - otherwise the plane usually starts to flick by the time they get to 45 degrees nose up!

The Sukhoi ailerons can be set up in any number of different ways with spade and gap-seal adjustments. They can be made very light or quite heavy, but the ailerons themselves are composite and lightweight - so they do not suffer the Yak's inertia problem.
A new aileron section was tried on the -31, but proved unpopular. Many -31s are now flying around having been retro-fitted with -26 ailerons.
The feel is just better.

Both aircraft have very effective rudders, but the Sukhoi's is just a bit more powerful. Combined with the big prop and neutral stability, the -26 and -31 are capable of awesome gyroscopics and this is where the unwary can get caught out.
It is easy to transition from negative snap to knife-edge tumble if the elevator is not kept sufficiently back during snap recovery.
The positioning must be quite precise.
If a downward snap to turns into a gyro with no warning, a lot of height goes by before you get your left hand back to shut the throttle and sort your feet out to stop the tumbling.
Once mastered, the Sukhoi's auto-freestyle characteristics are a source of endless amusement and countless bloodshot braincells, but in the initial stages of training - say the first 2 years - caution is the watchword, together with experienced supervision.

Conclusion:
You don't NEED a Sukhoi unless you really want a full-blown Unlimited steed and then you need lots of sound advice.
Anything less as your ambition and you'd do fine with a Yak-55.

Su-29:
The above deals with the main differences between the single seaters.
Two- seaters are always different, and I expect the Yak-54 shares the slight drawbacks shown by the -29.
The problem is mass distribution.
To get in the extra seat, you just have to move the pilot and engine further apart. The distance between the heads of the two pilots in a -29 is really quite large - no reaching forward and banging a wayward student over the cranium as in a Pitts. The result is increased longitudinal inertia. This makes all 2-seaters "sluggish" by comparison when rapid pitch and/or yaw inputs are required. This is also why an S2B is much harder to snap well than a S1S, or even an S2A. The same applies to the Russian aircraft.

In terms of performance, a -29 with one pilot fares almost as well as a -26, but you'll never find a Russian pilot flying one in the WAC!!