Hi,
I would strongly suggest getting a SIM that does have 4G capabilities, as 4G is much faster than 3G in most places. Also, in city centres, this will be very important, and I can vouch from experience.
When I was at De Montfort University, Leicester, I started off with Virgin Mobile. 3G only. Quite congested and got about 3Mbps download, and 0.5Mbps upload. But when I tried connecting my laptop to 3G (I didn't trust the wireless network at DMU with my own laptop), speeds went down to less than 1Mbps after a bit of use. In the end, 3G became unusable. Then switched to EE, which had 4G capabilities, and 4G was much more reliable and responsive. Speeds were good, generally about 20Mbps, which is good for a busy city.
I also went to an event in Lincolnshire earlier on in the year. It was very busy, so the network was under strain. I was with EE and had the option to lock to 3G, 4G or use the default mode (the latter, I never do since my S4 pushes me to 2G far too often).
When locked to 4G mode I was getting anywhere from 10Mbps to about 50Mbps, which is excellent considering the abnormally high load at that time. Coverage was very good on 4G as well; I had some sort of 4G signal everywhere during the event (apart from indoors sometimes, due to the lower power and higher frequencies of EE 4G).
When locked to 3G mode, however, things were very different. At the start of the day, 3-5Mbps was possible, however things quickly deteriorated and 0.5Mbps became the norm. On some occasions, there was no data at all and I had to switch to 4G only mode. Also, the coverage was far more hit and miss than 4G was; 3G has a property whereby the higher the load on the mast, the smaller the mast's coverage - so in some areas where 4G was available, 3G was either very weak or no signal at all.
Usually, this mast is fine on 3G and 4G, however, the high load meant that things were very different - and 4G seemed to cope much better than 3G.
My advice: get a PAC code from Talk Mobile and switch to a different provider, that offers 4G. If needed, you can get a SIM Only package and buy the phone outright. That way, you'll be covered is almost all circumstances. (A PAC code will allow you to transfer your existing mobile number to the new network.)
Hope that helps
edit: It should be noted that 4G will use your data faster than 3G, due to the faster speeds. Apart from a few things (such as speedtest.net, and streaming sites such as Spotify/Youtube/Tunein Radio etc, where the bitrate changes depending on your connection speed), the amount of data you will use on 4G will be the same as what you would use on 3G, assuming that you didn't change your usage habits when switching from 3G to 4G.
Also, something to bear in mind, make sure you do buy a phone that has 4G capabilities (most modern phones do) because, from what I have heard elsewhere, 3G may be switched off before 2G (though this still won't be for a good few years yet).