Personally if you likely to spend any time outside of Riga, then there is only one network to consider LMT. Tele2 are OK but limited in certain parts of the country and BITE (Vodafone) are relative newcomers and very patchy.. but cheap
In the UK, the cost of the phone is mixed in with the tarriff making you think you are getting a cheap phone.. but end up paying more over the contract period..
In LV they separate out the cost of the phone from the tariff plan, so many people buy their own unlocked phone and just pay the monthly tarrif. I presume you can get your Iphone unlocked
The LMT website details out many tariff and data options (no English version!) so you can choose what is suitable based on your phone, sms and data needs .. if you cant translate, then visit TRODEKS in Spice.. they speak English and will sort out your LMT needs 10 times faster than any LMT shop! (they are opposite the RIMI checkouts)
LMT do have good 4G coverage (even here in the countryside where I live) .. although check the cost versus 3G.. I get a minimum 2Mb/s on my 3G often 5Mb/s .. not your typical 256K in the UK..