1. Not using senior plan, but SingTel hi! S$13 plan (eSIM). You can take it as a reference.
In this particular case, 4G speed is pretty decent.
iPhone 14, 512GB version (my wife's main phone).
Good signal here in Jurong West area (near to Jurong Point)
1) 5G mode, 5G SA
https://www.speedtest.net/result/i/7159360994
2) 4G mode
https://www.speedtest.net/result/i/7159367017
2. BTW, SingTel S$15.28 (her main line on the same iPhone 14) performs very similar here since this is not a peak time and also with good signal.
1) 5G SA
https://www.speedtest.net/result/i/7159365880
2) 4G
https://www.speedtest.net/result/i/7159367921
3. The difference can be significant in other cases. I have tested Singtel hi! S$10 4G plan and I think it is not that good, worse than M1 Maxx 5G plan and StarHub Prepaid S 5G legacy plan.
If S$2 difference is okay for you, just go with the S$7 5G Senior plan. But you may want to compare with Gomo S$6 5G Senior plan as well.