Both of you are wrong. What India is doing is the "most" difficult of any kind of interception tests.
There are 3 stages where interception is possible. One is when the missile is in the boost phase, the second is when the missile is in the mid course stage and the third is the terminal stage.
Difficulty of missile interception is based on the speed of the missile or the warhead. So, the missile is the slowest during boost phase when the missile is only taking off slowly. This is the point where lasers like YAL-1 can be used to burn the electronics inside and is called boost phase interception. This is the easiest to achieve technologically. Even BVR missiles from aircraft can be used for this type of interception.
The next is mid course interception that China recently revealed. Aegis is also capable of mid course interception as this is the next best bet with the second highest success rate after boost phase interception.
The terminal stage interception that India has achieved is the toughest of the 3. This is because the warhead gives only a window of a few seconds in order to bring down a small target which is travelling at hypersonic speeds. It has the least success rate of the 3.
The countries with a boost phase interception capability as of today is the US with the YAL-1. The rest are trying to catch up and are far behind. The countries with capable mid course interception are US and Russia with the SM-2/3 and S-400/500 resply. China is currently testing their own missile and will take a few years to catch up. India has recently revealed technology capable of the same and will be some years away from induction as well.
As important as speed is required to break through for interception, the size of the target is also very important. The target is biggest during boost and smallest during terminal stage for obvious reasons. Small target size = greater difficulty in targeting.
The countries which have capable terminal interception are the US, Russia, Israel, France and India. The interception happens at an altitude of 200km and lesser and some examples are the US Patriot, the Russian S-300, the Israeli Arrow, French Aster-30 and Indian AAD/PAD.
India has 3 types of missiles for terminal stage interception. AAD which is tested against targets at an altitude of 30 to 70Km which is just above regular SAMs. AAD's equivalent is the Patriot, Arrow and Aster systems. PAD is meant for interception between 80 to 150Km high which is exo atmospehric. Then we have PDV which is a 2 stage SAM is a highly advanced variant of both AAD and PAD. It will handle all targets in the entire envelop of the BMD system and it's equivalent is the THAAD system of the US.
The reason why PDV is compared to THAAD is because of the radar system. While both AAD and PAD require a 600Km Swordfish radar, the PDV like THAAD will use a 1500Km Swordfish-2 radar system which will be highly mobile and highly robust system compared to AAD and PAD. Of course, both PAD and PDV can be used for mid course interception as well.
Lastly and most importantly is interception against ICBMs. As of today there is no technology that can intercept long range ICBMs in the terminal stage. It is actually impossible as of today because of processing and missile limitations. We have to have our interception missiles achieve speeds of Mach 10 to Mach 12 compared to Mach 4 to 6 that we have achieved today in order to target ICBMs and that's the biggest problem. We are still a few years away from achieving high speed SAMs.
The only countries facing an immediate ICBM threat are US, Russia, Europe, China and North Korea. So, they are currently trying to develop technology that can take out ICBMs during boost or mid course stage only because it is easier than taking out the high speed terminal stage warhead. That's why you see the above countries focus more on boost and mid course interception compared to terminal stage. On the other hand India does not face ICBM threats. So, we are more interested in terminal stage interception with the technology available today and slowly develop technology to intercept ICBMs in the long run. Similarly, even Israel is more interested in terminal stage interception because their enemies only have SR and IRBMs in their inventory. China has more reason to beat ICBMs than IRBMs. Also, Russia has already provided China with the S-300 system for terminal stage interception and denied the S-400 required for mid course interception and are therefore developing a S-400 system of their own.
Since now you know the differences between each stage and it's repurcussions on interception capability based on requirement, now you can make a better and more informed guess on which is harder to achieve. What China's done has not been particularly impressive like ours. But it is still a technology that has been tested before and is feared.
