The A51 is not the competitor to the new iPhone SE so such a comparison doesn't make much sense.
The appeal of the new iPhone SE is the small size and low price, so you need to compare apples to apples. The Samsung Galaxy S10e is the closest competitor to the Apple iPhone SE in terms of physical size and street price.
Compare the 128GB iPhone SE to the 128GB Galaxy S10E. The S10E has a street price about 20% less than the iPhone SE 128GB, even though the MSRP of the S10E is much higher. The iPhone SE is a little smaller; in height x width the iPhone SE is about 6% smaller than the S10E (though the iPhone SE is also 0.6mm thinner and weighs 2 grams less.
The S10E has many advantages over the SE including a bezel-less design with a high screen to body ratio, much larger screen, AMOLED versus LCD screen, higher resolution display, 3.5mm headphone jack, better waterproofing, MicroSD card slot, FM radio, USB-OTG support, more RAM, more cameras, higher-resolution cameras, 4x4 MIMO antenna, Magnetic Secure Technology (MST), much greater battery caoacity, and the Qualcomm X24 Cat. 20 LTE modem (versus the inferior Intel XMM7660 Cat. 19 modem in the iPhone SE).
One advantage of the iPhone SE is the dual-SIM (eSIM+Physical SIM) capability, which you don't get on any Samsung unlocked phone for the U.S. market. Processor-wise, neither would have any lag; the Apple A13 Bionic is very slightly slower than the Qualcomm Snapdragon 855 on the Geekbench Multi-Core benchmarks but much faster on Geekbench Single-Core benchmarks.
I emphasize "street price" because Samsung is aggressively promoting the S10E with large price breaks off MSRP. You can get a $250 trade in using an iPhone 6s and you can buy a new iPhone 6s for $100 to use as a trade-in! A used iPhone 6s in working condition is even less. Bottom line is that you can get the unlocked 128GB Galaxy S10E for under $400 with all the discounts.
I have a short check list for phones that I purchase these days and the S10E meets most all the criteria while the iPhone SE meets few of them:
✔ x Micro SD Card Slot
✔ x 3.5mm headphone jack
✔ x FM Radio
✔ x 1920x1080 minimum resolution
✔✔ 128GB minimum ROM
✔ x 4 GB minimum RAM
✔✔ Fast charging
✔✔ Fingerprint sensor
✔✔ Wireless charging
✔ x 5.8" or larger screen
✔ x Magnetic Secure Technology (MST)
✔ x Cat. 20 modem
What really interests me from Apple is the iPhone 12, if as recently revealed, they will have a model with a smaller physical size than the iPhone SE but with a bezel-less screen that is AMOLED, larger, and higher resolution. It will also have a Qualcomm X25 5G modem with 4x4 MIMO. It'll be a lot more expensive than the iPhone SE but that's okay. I'm done with low-end phones like the iPhone SE and I'll happily pay extra for a more capable phone with a higher-resolution, larger screen and better reception.
Covid-19 has also affected my phone preferences. I won't buy a new phone with only Face ID because when wearing a mask or face-covering it doesn't work. MST is desirable because so many places don't accept Apple Pay or Google Pay (Home Depot, Walmart, Lowe's, my HMO, and many smaller stores) but nearly every business takes Samsung Pay.
Hmm, I replied tonthis yesterday but my comment has vanished, same kinda view as above really.
In the UK the iPhone SE is £419 (almost no discounts anywhere). The A71 is £419 and A51 £319 at full price before the same sort of generous discounts mentioned by the OP. I bought my A71 with £70 cashback and £100 trade in for a worthless iPhone 4s! The A51 would have had £50 cashback and the £100 trade in.
iPhone with its ancient form factor for £419 vs either samsung at their 'real' price shows Samsung are getting it so right. Especially as we see that the iPhone chip is underclocked so not offering all the touted performance.