Should I get A1 or A2 spec'd Micro SD card for my phone?

TL;DR: Get an A1 card unless you know for sure that your workload greatly benefits from Command Queuing (CQ) and your host hardware supports CQ.

Long answer:

After looking around a lot it seems that the A2 specification could allow better performance but actual hardware implementations are slow enough that A1 is better choice nowadays (year 2019).

Thomas Kaiser has collected info about A1 and A2 rated SD cards and says

Application Performance Class 1 (A1)

This performance class requires at least 1500/500 read/write IOPS (IO
operations per second) with a 4k blocksize (small data chunks) and at
least 10 MB/s sustained sequential write performance. No special host
requirements are needed, the card simply has to exceed the performance
requirements on its own.

and

Application Performance Class 2 (A2)

A2 promises even better performance with 4000/2000 read/write IOPS
minimum but there's a problem since as outlined by the SD Association
A2 cards show "much higher performance than A1 performance by using
functions of Command Queuing and Cache".

Cache and Command Queuing require host (driver) support since the host
needs to activate those new features first. The cache feature on A2
rated cards makes use of volatile RAM on the card [...] involving the
risk of data losses [...]

Another user wrote about real world performance between Sandisk Extreme A1 and A2 cards:

I did some searching online and found a few other SD card benchmark
results where A2 cards performed worse than A1 cards, so my results
seemed to be what others were finding as well.

[...]
A2 cards not only require special hardware and/or software for maximum
performance, they actually perform WORSE than A1 cards without it

[...]
[Sandisk fine print about A2 cards]: Up to 160MB/s read speeds,
engineered with proprietary technology to reach speeds
beyond UHS-I 104MB/s, requires compatible
devices capable of reaching such speeds

[...]
As far as I can find, the only "compatible device" is the SanDisk
Mobilemate USB 3.0 micro SD card reader

James A. Chambers has tested A1 and A2 cards with Raspberry Pi:

A2 is not supported by Raspberry Pi yet (or almost anything else
really) so A1 is the important mark to hit.

So the current status seems to be that A1 is strictly about card hardware speed and A2 is about card + host hardware/firmware/software solution. The only bit about A2 which in fact could improve performance is Command Queuing (CQ) which allows card to process some requests slower and still allow higher average troughput even for small I/O operations. And if you care about worst case latency (QD1) then any queuing is never worth it so you do not even want this feature.

However, it seems that with current hardware (year 2019) of A2 cards the latency for any single request has been increased from A1 cards. I guess this is caused by too slow embedded processors inside the cards that try to do a bit too much. The firmware is probably optimized for benchmark throughput (the card can handle lots of requests in paraller but execute each request a bit slower) and as a result, the latency for any single request has gone up. As a result, if your host device doesn't support CQ feature of A2 cards, you get the worst of two worlds: the latency will go up and overall throughput will suffer.

In the end, if your host does not support A2 feature, get an A1 card. And if you care about QD1 performance, get an A1 card even if your host hardware/firmware + software does support all the A2 features.

According to latest benchmarks I would recommend either SanDisk Extreme A1 cards or A-Data Premier A1 cards. According to StorageReview benchmark, the A-Data Premier A1 card has highest random 4K performance: 2971 IOPS for 4K read, 1190 IOPS for 4K write. The SanDisk Extreme A1 cards is better known, though, so it may be a safer bet. The SanDisk Extreme A1 is already hard to find because SanDisk prefers to sell A2 variants now especially for bigger flash sizes.

I wouldn't be surprised if current crop of A2 cards were cheaper to manufacture than A1 cards because the manufacturers can use lesser spec'd internal processor to achieve the A2 minimum random access performance thanks to more buffering and queuing. I can only wish more benchmarking sites would focus on request latency over throughput.